fix: prevent panic on IOTA connection failure via lazy initialization

[nanderstabel]

Description of change

Restructures the Resolver to initialize IOTA network clients lazily instead of eagerly at startup.

Changes:

• Updated Resolver to use RwLock and double-checked locking for on-demand initialization.
  • Split iota_clients into a single-network get_iota_client helper. (ensures that one failing IOTA network (client) does not affect initialization of the other IOTA network client initialization)
  • Basic handlers (JWK, Key, Web) are now always available immediately.
  • Fixed an issue where IOTA connection failures could block non-IOTA DID resolution.
• Benefits:
  • Faster startup for non-IOTA use cases.
  • Improved resilience: unrelated IOTA errors no longer affect other DID methods.

Links to any relevant issues

Resolver::new panicking on startup in identity-wallet and ssi-agent.

How the change has been tested

Added unit tests:

• concurrent_resolutions_do_not_deadlock
• iota_failure_does_not_break_basic_resolver

Tested startup 'manually' in identity-wallet and ssi-agent.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[github-actions]

🎉 This PR is included in version 1.0.0-beta.8 🎉

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