feat(Gax): add stall control for resumable uploads

[bshaffer]

Adds stall control support to Resumable Uploads according to go/cloudsdk-scotty-stall-control.

Changes

• Adds transferStallMinimumRate (MiB/s) and transferStallTimeout (seconds) options to startUpload().
• Calculates chunk lag and tracks stall timeout clock across chunk transfers.
• Computes per-chunk timeouts and per-attempt timeouts (min(chunk_timeout, 2 * chunk_size / rate)).
• Preserves chunk deadlines across retry and recovery requests.
• Converts chunk deadline timeouts to an ApiException ("Upload stalled.") when global deadline is not set or not exceeded.
• Omits the default 10-minute global deadline when stall control is enabled.