fix: bytes field should be typed as a Buffer

[alvarowolfx]

protobufjs uses the $util.newBuffer function to decode and initialize bytes protobuf fields. This makes so that most of times I see those fields coming as Buffers, not Uint8Array or string. This was already observed by other users as you can see here and here.

This PR adds the Buffer type to the Typescript definition to take that into account.

Related issues/references:

• https://github.com/protobufjs/protobuf.js/blob/19e1fef36cc1b090a724f345b47a563af21c9506/cli/targets/static.js#L532
• protobuf wrong behavior on bytes protobufjs/protobuf.js#1482
• IAvroRows serializedBinaryRows typedef is a Buffer but defined as (Uint8Array|string|null) nodejs-bigquery-storage#335

Towards googleapis/nodejs-bigquery-storage#335 and internal b/277340507
Supersedes googleapis/nodejs-bigquery-storage#336

[sofisl]

This looks good to me, but ideally we'd run this test (or something like it) to confirm our downstream repos won't break with this change. If you have time @alvarowolfx, it would be awesome if we could run this test as part of changes to compileProtos! I'd be happy to pair with you if you want to add this test. Otherwise we can always just release an experimental version of gapic-tools and install it locally to confirm it doesn't break anything downstream.