Add wiki pages to reference filters framework

What does this MR do and why?

This adds cross-project and cross-namespace references for wiki pages using the extensible reference filters framework:

Type	Local	Cross-project	Cross-group
Wiki page	[wiki_page:Home]	[wiki_page:namespace/project:Home]	[wiki_page:namespace/group:Home]

While this is by itself of practical use to users, it will be used as a stepping stone for Wiki page mentions show as plain text instead o... (#550296 - closed).

References

• Add explicit cross-project references to projec... (#550305 - closed)
• Wiki page mentions show as plain text instead o... (#550296 - closed)
• Extensible reference filters (&7563)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a wiki page public-page (example name) in project_1.
2. Create a wiki page private-page in a private project_2, or in a project with a wiki with access restricted to project members.
3. Create a wiki page group-wiki-page in group_1.
4. Paste to some Markdown input in a project_1 work item:

[wiki_page:public_page]

[wiki_page:project_1:public-page]

[wiki_page:project_2:private-page]

[wiki_page:group_1:group-wiki-page]

<GDK URL>/project_1/-/wikis/public-page

<GDK URL>/project_2/-/wikis/private-page

<GDK URL>/groups/group_1/-/wikis/group-wiki-page

5. All the references should resolve to a wiki page link with its title as link text.
6. When logged in as a user that doesn't have access to project_2 wiki, links to page in the inaccesible wiki should not resolve.

MR acceptance checklist

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