SDK: Correctly encode auth header, and ser/de Credentials

[gefjon]

Description of Changes

Prior to this commit, the Rust SDK incorrectly encoded a Token passed to connect, making it impossible to re-connect an existing user. (Shows what I get for never testing that.) With this commit, auth tokens are correctly base64-encoded in an Authorization: Basic header with the username token, which allows re-connecting as an existing user.

Also, it was difficult to re-use Credentials from the Rust SDK, as:

• they were opaque types with non-exported members
• they did not implement any serialization / deserialization

This commit makes token.string and identity.bytes public fields, and implements SATN Serialize and Deserialize for Identity, Token and Credentials, allowing clients to save their credentials for re-use, e.g. to a file.

EDIT: A few more small changes, none of which felt important enough to be their own PR:

• subscribe_owned accepts Vec<String>; behaves like subscribe.
  • While developing letrs, a demo game, we needed to generate query strings at runtime, and passing them to subscribe would have involved an unergonomic dance of taking references which would then be converted to owned containers by the SDK. subscribe_owned eliminates that, allowing client authors to construct a set of queries at runtime and pass it directly to the SDK, without intervening referencies and copies.
• Docstrings for subscribe and subscribe_owned.
• Methods on Identity and Token for converting to/from strings and byte-vectors.
• Removed long-forgotten println within connect which wrote the computed URI to stdout.

API

• This is a breaking change to the module API
• This is a breaking change to the ClientAPI

If the API is breaking, please state below what will break

[kim]

What I ended up doing looks like this:

    fn client_credentials(&self) -> anyhow::Result<sdk::identity::Credentials> {
        let creds = SpacetimeCreds::encode_token(&self.private_key, self.identity)?;
        let token = axum::headers::authorization::Credentials::encode(&creds)
            .to_str()?
            .to_owned();
        Ok(sdk::identity::Credentials {
            identity: sdk::identity::Identity {
                bytes: self.identity.to_vec(),
            },
            token: sdk::identity::Token { string: token },
        })
    }

I wonder if it would be worth to have a dedicated spacetimedb-auth crate which handles these subtleties, and perhaps exports an opaque Token type?

[kim]

Linking clockworklabs/SpacetimeDBPrivate#526, as that will probably need to be updated to work with this patch

[gefjon]

Pushed another commit with some more minor changes. These are purely additions to the API or docs, so shouldn't effect Kim's work at all.

[jdetter]

These only really seems to add things to the API (doesn't change any existing funcitons) so I think we're good to go on this. I did a pass on testing the SDK so I tested some of this functionality but not some of the more specific parts (functions for getting the token, identity, etc.)