Add `GetIssueTemplates()` (#603)

This API was added in 1.16, [template forms](https://github.com/go-gitea/gitea/pull/20987) will be in 1.18.
I don't think we need to add version checks, as the respective `.Form` field will just be empty on unsupported versions, but correct me if I'm wrong here.

fixes #547

- i took the freedom to change the struct property names from the API names for clarity
- I didn't add  tests because it would effectively just test gitea's API to be functional  (as long as there is only this GET method)

Co-authored-by: John Olheiser <john+gitea@jolheiser.com>
Reviewed-on: https://gitea.com/gitea/go-sdk/pulls/603
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.io>
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>
Co-authored-by: Norwin <git@nroo.de>
Co-committed-by: Norwin <git@nroo.de>

gitea/issue_template.go

@@ -0,0 +1,97 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package gitea
+
+import (
+	"fmt"
+)
+
+// IssueTemplate provides metadata and content on an issue template.
+// There are two types of issue templates: .Markdown- and .Form-based.
+type IssueTemplate struct {
+	Name        string   `json:"name"`
+	About       string   `json:"about"`
+	Filename    string   `json:"file_name"`
+	IssueTitle  string   `json:"title"`
+	IssueLabels []string `json:"labels"`
+	IssueRef    string   `json:"ref"`
+	// If non-nil, this is a form-based template
+	Form []IssueFormElement `json:"body"`
+	// Should only be used when .Form is nil.
+	MarkdownContent string `json:"content"`
+}
+
+// IssueFormElement describes a part of a IssueTemplate form
+type IssueFormElement struct {
+	ID          string                      `json:"id"`
+	Type        IssueFormElementType        `json:"type"`
+	Attributes  IssueFormElementAttributes  `json:"attributes"`
+	Validations IssueFormElementValidations `json:"validations"`
+}
+
+// IssueFormElementAttributes contains the combined set of attributes available on all element types.
+type IssueFormElementAttributes struct {
+	// required for all element types.
+	// A brief description of the expected user input, which is also displayed in the form.
+	Label string `json:"label"`
+	// required for element types "dropdown", "checkboxes"
+	// for dropdown, contains the available options
+	Options []string `json:"options"`
+	// for element types "markdown", "textarea", "input"
+	// Text that is pre-filled in the input
+	Value string `json:"value"`
+	// for element types "textarea", "input", "dropdown", "checkboxes"
+	// A description of the text area to provide context or guidance, which is displayed in the form.
+	Description string `json:"description"`
+	// for element types "textarea", "input"
+	// A semi-opaque placeholder that renders in the text area when empty.
+	Placeholder string `json:"placeholder"`
+	// for element types "textarea"
+	// A language specifier. If set, the input is rendered as codeblock with syntax highlighting.
+	SyntaxHighlighting string `json:"render"`
+	// for element types "dropdown"
+	Multiple bool `json:"multiple"`
+}
+
+// IssueFormElementValidations contains the combined set of validations available on all element types.
+type IssueFormElementValidations struct {
+	// for all element types
+	Required bool `json:"required"`
+	// for element types "input"
+	IsNumber bool `json:"is_number"`
+	// for element types "input"
+	Regex string `json:"regex"`
+}
+
+// IssueFormElementType is an enum
+type IssueFormElementType string
+
+const (
+	// IssueFormElementMarkdown is markdown rendered to the form for context, but omitted in the resulting issue
+	IssueFormElementMarkdown IssueFormElementType = "markdown"
+	// IssueFormElementTextarea is a multi line input
+	IssueFormElementTextarea IssueFormElementType = "textarea"
+	// IssueFormElementInput is a single line input
+	IssueFormElementInput IssueFormElementType = "input"
+	// IssueFormElementDropdown is a select form
+	IssueFormElementDropdown IssueFormElementType = "dropdown"
+	// IssueFormElementCheckboxes are a multi checkbox input
+	IssueFormElementCheckboxes IssueFormElementType = "checkboxes"
+)
+
+// GetIssueTemplates lists all issue templates of the repository
+func (c *Client) GetIssueTemplates(owner, repo string) ([]*IssueTemplate, *Response, error) {
+	if err := escapeValidatePathSegments(&owner, &repo); err != nil {
+		return nil, nil, err
+	}
+	templates := new([]*IssueTemplate)
+	resp, err := c.getParsedResponse("GET", fmt.Sprintf("/repos/%s/%s/issue_templates", owner, repo), nil, nil, templates)
+	return *templates, resp, err
+}
+
+// IsForm tells if this template is a form instead of a markdown-based template.
+func (t IssueTemplate) IsForm() bool {
+	return t.Form != nil
+}