[WIP] Add Relying Party ID (RPID) validation

[Copilot]

RPID Validation Implementation Plan

• Explore repository structure and understand codebase
• Identify where RPID is used (MakeCredentialRequest, GetAssertionRequest)
• Create RPID validation function in proto/ctap2/model.rs
• Add validation to Ctap2PublicKeyCredentialRpEntity::new()
• Add validation to GetAssertionRequest for relying_party_id field
• Add comprehensive tests for RPID validation
• Test with valid RPIDs (example.com, sub.example.com)
• Test with invalid RPIDs (http://example.com, example.com:8080, etc.)
• Run existing tests to ensure no regressions
• Run linter and fix any issues
• Review and verify all changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Relying Party ID (RPID) validation</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Relying Party ID (RPID) validation #137

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