feat: add dynamic authorization support via callback provider

[EddieHouston]

Description

This PR adds support for dynamic authorization in Electrum RPC requests.
This enables authentication with authorization-protected Electrum servers and API gateways.

Use cases:

• Bearer token authentication (JWT, OAuth, API keys)
• API gateways requiring authorization headers
• Any dynamic authorization scenario where tokens need to be updated during the client's lifetime

Notes to the reviewers

The implementation uses a callback pattern rather than a static token to support dynamic scenarios like automatic token refresh. The AuthProvider is called for each RPC request, allowing the token to be updated without reconnecting the client.

Both Config and RawClient structs needed custom Debug implementations since function pointers don't implement Debug - this is handled by displaying <provider> when an auth provider is present, which prevents leaking sensitive token values in debug output.

Thread safety is ensured through Arc wrapping and Send + Sync bounds, making this safe for concurrent use across the client.

Tests added:
14 new tests, all passing

Changelog notice

Added: Dynamic authorization support via AuthProvider callback in ConfigBuilder. Enables authentication with authorization-protected Electrum servers and automatic token refresh patterns for Bearer tokens, API keys, and other authorization schemes.

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

concept ACK

Thanks for this, adding an "authorization" header is the most common way to authenticate to a http endpoints so this should be generally useful. Appreciate the comprehensive examples and unit tests.

The team is spread a bit thin so it may take some time to get it reviewed and tested.