feat: Complete buf export

[clockworkgr]

Description

buf includes well known types (e.g. google/protobuf/any, google/protobuf/descriptor etc.) implicitly. Hence buf generate does not need references to them.

However, buf export which is supposed to export all protos AND their dependencies to a specified folder (useful if you want to run some other proto tool on the SDK protos for whatever reason) assumes the same and does NOT export such well known types.

By explicitly including the protocolbuffers/wellknowntypes dep (https://buf.build/protocolbuffers/wellknowntypes) in buf.yaml, buf export will actually export the protos for those well known types as well.

This may be an edge-case scenario but seems useful (and was to me) so I thought a PR wouldn't hurt. Up to the maintaining team to decide if worth merging

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[julienrbrt]

I have no preference. So if that helps why not.