feat: Add support for returning backoff when stressed

[MikeGoldsmith]

Which problem is this PR solving?

Adds support for Refinery to respond to new telemetry requests with back pressure when stress relief has been activated.

For HTTP requests, Refinery will respond with HTTP status code 429 (TooManyRequests)
For gRPC requests, Refinery will respond with gRPC error code 8 (ResourceExhausted)

• Closes Apply back pressure to upstream service when Refinery is stressed #1553
• Similar to feat: Tell Load Balancers to Back Off #1530

Short description of the changes

• Add StressRelief config option DisableBackPressure
• Add ErrBackPressure error
• When handling incoming data (event, batch or OTLP) check if the collector processed is stressed, if yes, then return new error
• Check for new error at request handler entry point, and return appropriate error
• Add unit tests to verify behaviour