[STAC] Inject `titiler` links in collections/Items/search

[vincentsarago]

After #26 we will have the full Client customized, meaning that it will be easy to change the Links Models (https://github.com/stac-utils/stac-fastapi-pgstac/blob/main/stac_fastapi/pgstac/models/links.py)

e.g

from stac_fastapi.pgstac.models import links 

@attr.s
class CollectionLinks(links.CollectionLinks):

    def link_titiler(self) -> Dict:
        """Create the `titiler` link."""
        return {
            "rel": Relations.data.value,
            "href": f"{settings.titiler_endpoint}/collections/{self.collection_id}/tiles",
        }