Add granular pats decorator for wiki REST endpoints

What does this MR do and why?

Make following endpoints granular PATs compliant

Group endpoints

GET /groups/:id/wikis
GET /groups/:id/wikis/:slug
POST /projects/:id/wikis
PUT /groups/:id/wikis/:slug
DELETE /groups/:id/wikis/:slug
POST /groups/:id/wikis/attachments

Project endpoints

GET /projects/:id/wikis
GET /projects/:id/wikis/:slug
POST /projects/:id/wikis
PUT /projects/:id/wikis/:slug
DELETE /projects/:id/wikis/:slug
POST /projects/:id/wikis/attachments

References

#574463

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In Rails console, create a granular PAT for a user and copy a URL to create an issue with the token:

# Enable feature flag
Feature.enable(:authorize_granular_pats)

user = User.first

# Create granular token
token = PersonalAccessTokens::CreateService.new(
  current_user: user,
  target_user: user,
  organization_id: user.organization_id,
  params: { expires_at: 1.month.from_now, name: 'gPAT', granular: true }
).execute[:personal_access_token]

# Get a project, ensure they have avatars
project = user.projects.first

# Add granular scopes for read_avatar permission (one at a time)
project_scope = Authz::GranularScope.new(namespace: project.project_namespace, permissions: [:read_wiki])

Authz::GranularScopeService.new(token).add_granular_scopes(project_scope)

# Copy project avatar URL
IO.popen('pbcopy', 'w') { |f| f.puts "curl http://#{Gitlab.host_with_port}/api/v4/projects/#{project.id}/wikis -X GET -H \"PRIVATE-TOKEN: #{token.token}\"" }

2. Similarly test for other endpoints mentioned in description.

MR acceptance checklist

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Related to #574464 (closed)