protoc-gen-openapiv2: Fix so that FieldMasks descriptions are correctly generated

[krosen040]

References to other Issues or PRs

Fixes #1556

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Added google.protobuf.FieldMask as a well-known-type in protoc-gen-openapiv2 package so that fields of this type are generated with the correct description in Swagger definitions.

Other comments

This also had some other side effects, which I'm not sure are considered breaking changes.

• The name of the FieldMask fields in the Swagger definition will no longer be suffixed with .paths
• The protobufFieldMask type definition will no longer be added to the Swagger definition

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[krosen040]

@googlebot I signed it!

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[krosen040]

@googlebot I fixed it.

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[johanbrandhorst]

Thanks for your contribution!