Add .proto to packages of Protobuf generated Java classes

[woop]

What this PR does / why we need it:

Adds .proto to the package of Java generated proto code in order to differentiate it from non-proto code.

Example: feast.core.CoreServiceGrpc to feast.proto.core.CoreServiceGrpc

Which issue(s) this PR fixes:

Fixes #684

Does this PR introduce a user-facing change?:

Java classes generated from Protobuf files have had .proto added to their package path.

[zhilingc]

/test test-end-to-end-batch

[woop]

/test test-end-to-end-batch

[woop]

/test test-end-to-end-batch

[ches]

/lgtm

Hope there isn't a reason to hold this—there's no hold label—but it's potentially a big merge conflict pain waiting for anyone who branches before this gets in, so if we want it let's get it done. Anyone with already open PRs, yell at @woop not me 😉

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ches, woop

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[woop]

/lgtm

Hope there isn't a reason to hold this—there's no hold label—but it's potentially a big merge conflict pain waiting for anyone who branches before this gets in, so if we want it let's get it done. Anyone with already open PRs, yell at @woop not me

I think there will be yelling, but it won't be because of this :)

We've had a long running Mexican standoff between PRs for a while now, so I have asked @mrzzy to just squash his history into a single commit (well it's 4 now) so that he doesn't lose years of his life rebasing.

Anyway, thanks for getting this merged in!