UCP/CONTEXT/CONFIG: create env vars to force MULTI_SEND and FAST_CMPL

[michal-shalev]

What ?

Create environment variables (UCX_FORCE_FAST_CMPL and UCX_FORCE_MULTI_SEND) to automatically enforce the UCP_OP_ATTR_FLAG_FAST_CMPL and UCP_OP_ATTR_FLAG_MULTI_SEND flags for all UCX operations.

Why ?

These variables provide a way to globally enforce specific behaviors (FAST_CMPL and MULTI_SEND) across all UCX operations, optimizing performance without needing to modify each operation individually.

How ?

Added FORCE_FAST_CMPL and FORCE_MULTI_SEND in ucp_context_config_t and updated environment parsing logic. Updated functions to check these new settings and apply the flags accordingly.