Migrate google-cloud-java/google-cloud-datastore to use GAPIC low-level libraries

[wsh]

Right now, https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-datastore depends on https://github.com/GoogleCloudPlatform/google-cloud-datastore, which is suboptimal for several reasons:

1. No support for gRPC.
2. Lack of uniformity with other google-cloud-java clients.
3. Continued dependence on the separate google-cloud-datastore artifact, which frequently leads to confusion because of the unfortunate naming similarity. We'd like to deprecate this artifact altogether.

[wsh]

cc @dmcgrath @lesv @pcostell @eddavisson

[lesv]

@garrettjonesgoogle FYI
@jabubake This probably falls in your world now.

[anthmgoogle]

This is a pretty significant investment. We should consider it in a next round of triage.

[yihanzhen]

Move this issue to feature backlog and close it here. The place can continue to be used for comments and discussion.

[wsh]

...since this is being closed and moved, do we have any indication when it might be acted upon? cc @dmcgrath

[pongad]

@wsh How much/urgently do you want this? @hzyi-google and I are working to burn down "granular" bugs in the queue. That's taking a little longer than we anticipated.

My current plan is to get to longer-term requests like this one after the granular bugs are burned down. If you'd like to keep this open to make sure we don't lose it, that's fine with me.

Does this sound OK to you?

[wsh]

@pongad it is a high priority for us (our most important ask for google-cloud-java, at least), but I can't speak to how that balances with the other priorities you all have. Keeping this open would be nice for tracking.

[meredithslota]

Updating based on moved links: https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-datastore has been moved to this repo, https://github.com/googleapis/java-datastore.

The low-level Java code that used to live in https://github.com/googleapis/google-cloud-datastore/tree/main/java has been migrated to https://github.com/googleapis/java-datastore/tree/main/datastore-v1-proto-client, which I think was the core request in this issue. This happened in googleapis/google-cloud-datastore@c6c5001 (removed stale code) and the migration into this repo was done earlier in #427.

I think we're ok to close this out, but please let me know if we missed something.