Add extra_setup_config escape hatch for on-prem setup message (#836)

The on-prem enclave setup message gains fields faster than this SDK
models them: `bedrock_inference_profile` has been read server-side
(xi_on_prem/enclave/routes/conversation.py) since the global Bedrock
inference profiles landed, but there was no way to send it from here.

Add `extra_setup_config`, a dict merged as-is into the setup message, so
new server-side fields no longer require an SDK release. Keys that
collide with the typed fields are rejected at construction time, so an
escape-hatch value can never silently defeat the validation on those
fields (e.g. the transcription/audio webhook mutual exclusion).

Also add `bedrock_inference_profile` as a typed field, since it is a
known, shipping parameter that deserves discoverability; it is omitted
from the message when unset, leaving the server default of "us".

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>