Avoid $ref in /queryables

[m-mohr]

Neither a bug nor really a feature, so opening a blank issue.

The /queryables endpoints refer to external schemas using $ref. This leads to a lot of downstream HTTP requests in clients, e.g. STAC Browser. For example requesting https://planetarycomputer.microsoft.com/api/stac/v1/queryables has a single field defined, which pretty much just refers to a very simple JSON Schema of like 30 bytes: { "type": "string", "minLength": 1 }. For this simple schema. the client has to sends 8(!) http requests returning a total of nearly 10.000 bytes. Could this be improved?

Two alternative solutions could be considered:

1. Inline at least the simple schemas.
2. Bundle the schema on the server-side and return the bundled schema.

I'd prefer version 2 as it just is much more lightweight on the clients, which don't need to carry a full $ref parser library. $refs itself are also pretty ill-defined with a lot of different implementation, so the interoperability is pretty bad. The JSON Schema community is currently discussing this, but that will take more time: https://github.com/json-schema-org/referencing https://phil.tech/2022/bundling-openapi-with-javascript/