libfabric-2.0: Add hints input and caps output to collective join

[j-xiong]

Collective offload may have optional features that can be enabled upon request. It is desirable to be able to pass hints to the collective join function and get back the information about the optional caps.

[j-xiong]

an example of the hints is the set of collective operations to be performed on the collective group.

[j-xiong]

Double checked the man pages and the "experimental" label has been removed from the collective API description since v1.18.0. That means we have to introduce a new API call for this.

[j-xiong]

See #10317

[j-xiong]

Removed from #10317.

The main objective is to tell the provider the set of collective operations that are going to be performed on the group. That information is already part of the av_set attribute when the av_set is created.

[j-xiong]

No longer needed.