Support for configuring the browser fetch credentials mode in BrowserClient.

[mauriceraguseinit]

This PR adds support for configuring the browser fetch credentials mode in BrowserClient.

Currently BrowserClient only exposes a boolean withCredentials option, which maps to either same-origin (when false) or include (when true). The browser fetch API, however, supports a third credentials mode, omit, which cannot be expressed through the existing boolean API. The only way to use omit today is to fork or copy BrowserClient and change the RequestInit manually.

Closes #1936.

What this changes

• Adds a new BrowserCredentialsMode enum with the values omit, sameOrigin, and include, matching the three credentials modes defined by the [Fetch standard](https://fetch.spec.whatwg.org/#requestcredentials).
• Adds a credentialsMode property and constructor parameter to BrowserClient, defaulting to BrowserCredentialsMode.sameOrigin to preserve the existing default behavior.
• The RequestInit.credentials value is now derived from credentialsMode.

Backward compatibility

• The existing withCredentials property is kept and continues to work, but is now deprecated because a boolean cannot represent all three credentials modes.
• It is implemented as a compatibility adapter on top of credentialsMode:
  • the getter returns true only when credentialsMode is include;
  • the setter maps true to include and false to sameOrigin.
• Existing code such as BrowserClient()..withCredentials = true keeps producing the same fetch behavior as before.

New usage

final client = BrowserClient(
  credentialsMode: BrowserCredentialsMode.omit,
);

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