feat: gRPC mempool service (backport #275)

[mergify]

This is an automatic backport of pull request #275 done by Mergify.
Cherry-pick of 7d8a695 has failed:

On branch mergify/bp/release/v1.x.x/pr-275
Your branch is up to date with 'origin/release/v1.x.x'.

You are currently cherry-picking commit 7d8a695.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .gitignore
	modified:   block/mempool.go
	modified:   block/mempool_test.go
	new file:   block/service/README.md
	new file:   block/service/service.go
	new file:   block/service/service_test.go
	new file:   block/service/types/query.pb.go
	new file:   block/service/types/query.pb.gw.go
	new file:   proto/sdk/mempool/v1/query.proto
	modified:   tests/app/app.go
	modified:   tests/integration/chain_setup.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Makefile
	both modified:   tests/integration/block_sdk_suite.go
	both modified:   testutils/utils.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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