(sdk: local_relay): Add relay_url to LocalRelayBuilderNip42

[zupzup]

Description

Since the nip42 authentication now also checks the relay tag and defaults to self.url().await, this means it validates against the internal URL of the relay, instead of the public-facing URL.

This is fine for tests, but problematic for all other use-cases.

This PR adds an optional relay_url to LocalRelayBuilderNip42, which lets the user override this behaviour, so the validation is against their defined public URL.

Checklist

• I followed the contribution guidelines
• I updated the relevant CHANGELOG.md (if applicable)
• I understand and can explain all code in this PR

[TheAwiteb]

I would removed this extra 3 lines, I don't see them useful

[zupzup]

Removed and fixed the cargo fmt issue from CI 👍

[TheAwiteb]

ACK

[codecov]

Codecov Report

❌ Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
nostr-sdk/src/local_relay/builder.rs	14.28%	6 Missing ⚠️
nostr-sdk/src/local_relay/local/inner.rs	85.71%	1 Missing ⚠️

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