Issue 1: good idea?

Is this a good idea?

We are defining some models in `tipg`, `morecantile` and `titiler` but sometime we end up duplicating the code, so I'm leaning toward having all the models in one place 🤷

We have seen with `stac-pydantic` that it might not have been a great idea, because the maintenance burden it created 🤷

Begin by reviewing the existing model definitions in tipg, morecantile, and titiler, then compare the maintenance experience of stac-pydantic. No files, tests, or concrete acceptance criteria are named; this issue is complete only once the shared-model direction and its scope are agreed.

Written by the indexing model from the issue text.