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

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

```python
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",
}

```