UCP/PROTO: Protocol variants

[iyastreb]

What?

This PR introduces protocol variants for multi-protocols.
The problem: with existing implementation is that for multi-protocols selection we always prefer high-bw combination of lanes (for instance cuda_ipc), which might not be the best choice on low message dimensions. Because high-bw protocols often have higher latency.
This PR attempts to solve this problem by introducing protocol variants, so that for a single multi-protocol we may select multiple protocols: currently high-bw lanes selection and low-latency lanes selection.

Why?

Improve performance of multi-protocols on low message dimensions.

How?

• Added infra for protocol variants with a possibility to extend
• Added latency variant in addition to existing bandwidth one
• Moved protocol selection logic to proto_common, so that we can use it for single protocols as well
• Added UCX_PROTO_VARIANTS option that enables this feature with default value n
• Allocate selection on heap, added memory pool for that

TODO in next PRs:

• Enable UCX_PROTO_VARIANTS=y by default
• Use protocol variants in single protocols
• Long-living task: implement proper aggregation of multi-path selection (according to single path BW)

Testing results