Implement Agent Workload API Rate Limiting and fix rootless image builds

[Pittu-Sharma]

• Commit conforms to CONTRIBUTING.md?
• Proper tests/regressions included?
• Documentation updated?

Affected functionality
Workload API (Agent), Server Middleware, and Image Build process.

Description of change
This PR adds rate limiting to the Agent Workload API and fixes failures when building images in restricted environments (e.g., rootless DinD).

Key updates:

• Shared Ratelimit: Implementation refactored into a shared pkg/common/ratelimit package. It provides a thread-safe map with background garbage collection for inactive limiters.
• Agent Middleware: Implemented gRPC interceptors for the Workload API. The middleware extracts caller identity (Pod UID on Linux, falling back to PID) to enforce per-workload limits.
• Agent Configuration: Added workload_api_rate_limit to the agent configuration (HCL and CLI).
• Cross-Platform Support: Platform-dependent logic is separated into ratelimit_posix.go and ratelimit_windows.go to ensure consistent builds across Linux and Windows.
• Makefile Improvements: Updated the container-builder target to check for existing builders and introduced a BUILDX_BUILDER variable. This prevents failures in rootless environments where the default docker-container driver may lack necessary permissions (make images fails for rootless docker in docker #5140).

Which issue this PR fixes
Fixes #5140

[sorindumitru]

Hi @Pittu-Sharma, thanks for opening this PR. There's already one for this issue open, #6724, which is now going through some review cycles. We'll close this one in favour of that one.