Add AuthCtx to ReducerContext for rust

[jsdt]

Description of Changes

This exposes client credentials in reducer calls for rust.

API and ABI breaking changes

API Changes:

The main API change is the addition of AuthCtx and the sender_auth in ReducerContext.

ABI Changes:

This adds two new functions jwt_len and get_jwt. These use st_connection_credentials to look up the credentials associated with a connection id. jwt_len can be used to figure out the size of the payload, so that get_jwt can provide a large enough buffer.

TODO: I assume I need to bump the ABI version in this PR.

Expected complexity level and risk

2. This adds new ABI functions

Testing

I've done some manual testing with modified versions of the quickstart. We should add some examples that use the new API.

[Centril]

Looks like there were also stuff from my previous review that was not addressed.

[Centril]

I have some improvements atop of this PR, primarily wrt. docs, but this looks good to merge.