feat(everest): add client for database credentials

[amh1k]

Description

Adds an OpenEverest API client to retrieve connection details for a PostgreSQL instance using its registered Kubernetes cluster name, namespace, and instance name. These details will support the coordinator’s future runner credential setup.

Changes

• Forwards the caller’s authorization token to OpenEverest.
• Returns structured connection details and validates required fields.
• Applies a 10-second HTTP timeout and propagates caller cancellation.
• Adds typed errors for request failures, response-read failures, invalid responses, and unsuccessful HTTP statuses.
• Excludes upstream error response bodies to avoid exposing credentials.

Testing

Tests cover successful retrieval, authorization failures, missing targets, invalid or incomplete responses, upstream failures, body-read errors, timeouts, and cancellation.

cd backend && go test ./...

Note

The client returns the credentials based on namespace, cluster and instance name. It currently does not have the functionality to only return PostgreSQL databases

Kubernetes Secret creation, Job execution, and benchmark API handlers remain separate work.

Closes #9