Move GCS+gRPC plugin to v2 protos

[coryan]

The v2 protos for GCS+gRPC were just published:

https://github.com/googleapis/googleapis/tree/master/google/storage/v2

We would like to move the GCS+gRPC plugin to use these protos. I anticipate at least the following steps:

• Pick a new minimum version of Clang (DONE: 6.0) and create a build for it
• Remove the clang-38 build and update documentation to say Clang-6.0 is the new minimum
• Update the googleapis baseline.
• Update all the builds to require Protobuf >= 3.15
• Add the new protos to the storage_protos library.
• Remove the functionality not available in v2/ protos.
• Remove any tests or integration tests using functionality (e.g. metadata operations) that these protos do not implement.
• Change the emulator to use the new protos.
• Rewrite the StorageStub (and friends) and the internal::GrpcClient to use these new protos.
• Figure out what to do with the QuotaUser field.

[coryan]

I picked Clang-6.0 as the new minimum because it is the earliest default Clang in any of the distros we support. Ubuntu:16.04 ships with Clang-3.8, and Ubuntu:18.04 ships with Clang-6.0... So 6.0 it is.

[vnghia]

Hi @coryan , please ping if there is anything I can help on this issue 😄

[coryan]

@vnvo2409 thanks for the offer. Please be patient with us, I just deleted a lot of the code you wrote, but we will add it back (with some -- hopefully minor -- changes).

[coryan]

Okay, I figured out the answer to quota user, we need to add this as as x-quota-user header in the grpc::ClientContext:

https://cloud.google.com/apis/docs/system-parameters