PLUGIN/UCX: Init only needed fields in UCP (#2017)

## What?
There is a visible 2% perf degradation in nixlbench (512Bx64000 8
threads) on EOS and DFW

## Why?
UCP SGL PR https://github.com/openucx/ucx/pull/11258 increased the size
of request param, which is zero-initialized in NIXL on each operation.
This extra initialization costs 2% of performance.

## How?
We need to explicitly set only fields that we populate, no need to
zero-init the rest


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## Summary by CodeRabbit

* **Performance**
  * Improved handling of low-probability transfer failures.
* Streamlined internal transfer request setup for send, read, and write
operations.

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