feat(client): Accept optional tokens in ClientBuilder::token

[jan-auer]

The Client builder supports chaining methods. However, there was no way to conditionally supply a token generator while chaining builder methods, which is unergonomic.

ClientBuilder::token now accepts an Option directly via a new IntoTokenProvider trait. A None leaves the client unauthenticated. The new trait is also implemented for all types that were accepted previously, keeping backwards compatibility.

This brings the Rust client in line with the Python client, which already accepts an optional token:

// before
let mut builder = Client::builder(remote);
if let Some(token) = token_opt {
    builder = builder.token(token);
}
let client = builder.build()?;

// after
let client = Client::builder(remote)
    .token(token_opt)
    .build()?;

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.41%. Comparing base (2cc842a) to head (d1214c7).

Files with missing lines	Patch %	Lines
clients/rust/src/auth.rs	77.77%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   87.40%   87.41%           
=======================================
  Files          86       86           
  Lines       13893    13892    -1     
=======================================
  Hits        12143    12143           
+ Misses       1750     1749    -1     

Components	Coverage Δ
Rust Backend	92.28% <ø> (ø)
Rust Client	80.07% <81.81%> (+0.04%)	⬆️
Python Client	88.51% <ø> (ø)

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[lcian]

Looks good.