Add titiler-pgstac functions to API

[gadomski]

• geojsonsearch
• search_query
• list searches
• readonly

cc @vincentsarago

[vincentsarago]

just looking at titiler-pgstac, I have also some need to list all the registered searches: https://github.com/stac-utils/titiler-pgstac/blob/d0d2f868801f3c47b0a34f70472687aec3a32cbd/titiler/pgstac/factory.py#L1545-L1566

and also pgstac.readonly() function https://github.com/stac-utils/titiler-pgstac/blob/d0d2f868801f3c47b0a34f70472687aec3a32cbd/titiler/pgstac/dependencies.py#L126-L130