Rust SDK: with_credentials -> with_token

[gefjon]

Description of Changes

See discussion on clockworklabs/com.clockworklabs.spacetimedbsdk#212

This PR replaces DbConnectionBuilder::with_credentials with ::with_token, which does not require an Identity as argument. It also amends our machinery for saving and loading credentials to and from disk to suit the new API.

API and ABI breaking changes

Yep!

Expected complexity level and risk

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• I believe automated tests to be sufficient.

[RReverser]

I'm not a fan of impl ToString since this accepts almost any type in Rust ecosystem (any type that implements Display), eg user can accidentally pass Some(true) as a token and it will be silently stringified, but it's up to you.

No concerns otherwise.

[gefjon]

I'm not a fan of impl ToString since this accepts almost any type in Rust ecosystem (any type that implements Display), eg user can accidentally pass Some(true) as a token and it will be silently stringified, but it's up to you.

No concerns otherwise.

We use impl ToString elsewhere. I'm not aware of any other trait in std that means "string-like thing that can be converted to String."

[RReverser]

We use impl ToString elsewhere. I'm not aware of any other trait in std that means "string-like thing that can be converted to String."

Literally impl Into<String> is much better IMO. It covers all the usual suspects - String, &str, Cow<str> etc.

[gefjon]

Literally impl Into is much better IMO. It covers all the usual suspects - String, &str, Cow etc.

If you want to open a follow-up PR that changes all uses of impl ToString in DbConnectionBuilder, I will happily accept it.