Refactor EXLA Client to use PjRt

[seanmor5]

This PR refactors EXLA to use PjRt. Once merged this will accomplish multiple things:

• Bumps to stable TF 2.5. Resolves Pin EXLA to a TF Stable Release #62
• I believe there has been progress upstream on MAC ARM, so it possibly resolves XLA doesn't support Mac ARM #217
• Makes for far more straightforward asynchronous execution e.g. by exposing BlockHostUntilReady, which will make resolving Introduce Nx.Stream #200 much easier
• Really reduces the EXLA NIF code footprint so it will be MUCH easier to maintain moving forward
• Introduces a TPU Client for using Nx with GCP TPU Hosts

As is, fails all async tests on CPU; however, we may want to consider adjusting how async works considering how much simpler this makes things. It also fails two op tests: QR Decomposition and Window Max. I'm investigating whether or not this might be an upstream issue.

Fails most tests on TPU. This is an issue with the executable not liking the layout of our input buffers. I've been talking with some of the PjRt developers about this issue.

Yet to test on GPU, but I don't think there will be any major hiccups.

I will need to adjust our Makefile to try and get rid of the NumPy dependency before this is merged.