Async flavored client

This thought came up when considering if we can use this library in our STAC API to talk to Data Auth API (/signup/api/users/{username}/ and /signup/api/users/{username}/) as part of adding authorization. Some of the response models aren't complete enough for the needs, but the bigger blocker is that it'd be unfortunate to put sync code into async endpoints.

It seems like there's at least a few options:

1. Write an AsyncCsdaClient alongside the (Sync)CsdaClient. We might consider refactoring the internals per endpoint into some "core" for URL and payload construction that each flavor uses
2. Write an AsyncCsdaClient and use unasync to rewrite AST and create the (Sync)CsdaClient as part of development process (e.g., put it in CI).
   • This is what the official Python ElasticSearch client does
3. Write the AsyncCsdaClient and convert our (Sync)CsdaClient to delegate to it via asyncio.run() (I think this can blow up in contexts with an existing event loop)

Start by reading the existing SyncCsdaClient and the endpoint handling for /signup/api/users/{username}/. Compare the three proposed approaches—an AsyncCsdaClient, unasync generation, or asyncio.run() delegation—and check the response models needed by the STAC API use case. Done means an agreed implementation strategy and an async client that supports the required authorization calls without disrupting sync usage.