Adding conditional passing of metadata to devices

[ram-mac]

Arista devices expect metadata to have username, password even in mTLS scenario.
So, this change modifies GetRequestMetadata to conditionally omit the "password" from the returned metadata when using mTLS with Arista devices.
This change ensures that Arista devices, which extract the username from the certificate in mTLS mode, do not fail due to an unexpected password in the metadata.
The vendor is now passed to PassCred to enable this vendor-specific logic.

[gemini-code-assist]

Summary of Changes

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This pull request implements a necessary adjustment for Arista device compatibility within the gRPC communication layer. By introducing vendor-aware logic into the credential handling, the system can now conditionally suppress password metadata during mTLS sessions, ensuring that devices relying on certificate-based identity do not reject connections due to extraneous authentication headers.

Highlights

• Vendor-specific metadata handling: Introduced a vendor field to the creds struct to allow for device-specific logic during gRPC metadata generation.
• Arista mTLS compatibility: Modified GetRequestMetadata to conditionally omit the password field when communicating with Arista devices over mTLS, preventing authentication failures.
• Metadata override logic: Refactored the metadata generation to respect existing context values and avoid redundant or conflicting username entries.

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[OpenConfigBot]

Pull Request Functional Test Report for #5297 / b84c812

No tests identified for validation.

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[gemini-code-assist]

Code Review

This pull request updates the gRPC credential logic to include vendor-specific handling, specifically for Arista devices. However, the current implementation contains several critical issues, including a compilation error due to an undeclared variable returnMap, typos in field and constant names, and hardcoded vendor logic that hinders multi-vendor support.