[Epic] Add support for `mhlo.custom_call`

[rsuderman]

There are some operations that we will likely support via library calls (e.g. QR, Eigh, etc). We should add support for a method to call out to related libraries to execute this solvers as they may not map well to the linalg code generation system.

(Everything below is EDIT by @vmurali )

This is the initial design document for this feature.

• [x] #13172
• [x] #13162
• [ ] #13164
• [x] https://github.com/google/jax/issues/15357
• [ ] #13163
• [ ] #13165
• [ ] #13166
• [ ] #13167
• [ ] Profit

[allieculp]

@vmurali Can you help update this Epic?

[vmurali]

Finished first task and trying to finish the second task.

[aaron-schneider]

(Updated priority.)

[allieculp]

@rsuderman Cleaning up issues - is this still active/relevant?