Update merchant authentication to handle multiple merchants on one Segment instance

[jramorris]

A summary of your pull request, including the what change you're making and why.

Previously a single merchant authentication was cached, causing the same token to be used for multiple merchants when running on the same instance. This update sets a cached token per merchant.

Testing

Include any additional information about the testing you have completed to
ensure your changes behave as expected. For a speedy review, please check
any of the tasks you completed below during your testing.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.

Security Review

Please ensure sensitive data is properly protected in your integration.

• Reviewed all field definitions for sensitive data (API keys, tokens, passwords, client secrets) and confirmed they use type: 'password'

New Destination Checklist

• Extracted all action API versions to verioning-info.ts file. example

leaving this unchecked as this is not a new destination