fix(key-auth): fix the issue of credentials reading failure

[NelZyhh]

Ⅰ. Describe what this PR did

This PR enhances the key-auth Go plugin to align with the C++ implementation and introduces more flexible configuration options for consumers.

Key Changes:

1. Support for Multiple Credentials : Added the credentials field to the Consumer configuration. This allows a single consumer to possess multiple valid API keys (tokens).
2. Consumer-Level Customization : Enabled keys , in_query , and in_header configurations at the consumer level. This allows specific consumers to use different API key names or extraction sources (header/query) independent of the global settings.
3. Authentication Logic Alignment :
   • Removed the limitation that rejected requests containing multiple tokens (previously returning deniedMultiKeyAuthData ).
   • Updated the authentication flow to iterate through all extracted tokens. The request is now successfully authenticated if any valid credential is found among the tokens.
   • This change ensures the behavior is consistent with the C++ version of the key-auth plugin.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

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