Support auth/signing metadata for streaming upload calls

[Marenz]

Problem

Currently, the base client only attaches auth and signing metadata (key, timestamp, nonce, signature) for:

• Unary-to-unary calls
• Unary-to-stream calls

It does not attach this metadata for stream-to-unary or stream-to-stream calls (streaming uploads).

Impact

Clients that use streaming upload calls (e.g., UpsertMarketLocationSamplesStream in the market-metering client) need to manually attach the auth/signing metadata, leading to code duplication and potential bugs.

See workaround in frequenz-client-marketmetering-python PR #48: frequenz-floss/frequenz-client-marketmetering-python#48

Proposed Solution

Extend the base client to automatically attach auth/signing metadata for all call types:

• Unary-to-unary ✓ (already supported)
• Unary-to-stream ✓ (already supported)
• Stream-to-unary (needs support)
• Stream-to-stream (needs support)

This would require handling the metadata injection differently for streaming calls, possibly:

1. For stream-to-unary: Attach metadata before starting the request stream
2. For stream-to-stream: Attach metadata when starting the stream

References

• frequenz-client-marketmetering-python workaround: Fix auth for sample upsert stream frequenz-client-marketmetering-python#48