Add a way for users to inject their own links into API responses

What's the status of this PR? IMO stac-fastapi shouldn't host its own tiler - the titiler-pgstac is a good example of standing up a tiler that uses STAC but a separate service that can be deployed independently. In our implementation of stac-fastapi, we override the pgstac backend core client in order to iterate over collections/items and inject tiler links where appropriate. I think this approach allows stac-fastapi to concentrate on just the STAC API endpoints, but allows customization by users if they want to add titiler or other tiling capability. I could see work done to generalize and implement a link injection mechanism so that other libraries can write hooks into the STAC objects returned by stac-fastapi so that the link/asset injection logic can live in that external project. But I don't think we should create tilers directly in stac-fastapi.

Originally posted by @lossyrob in https://github.com/stac-utils/stac-fastapi/issues/97#issuecomment-886217367

No files, tests, or entry points are named. Start by reviewing how stac-fastapi builds API responses and how the pgstac backend core client is overridden; done means external projects can inject links or assets through a generalized mechanism without stac-fastapi hosting a tiler.