Gmail API: ETags are not supported in the ruby library, but they are supported in the API

In the GMail API, ETags are supported for Filters:

`https://developers.google.com/resources/api-libraries/documentation/gmail/v1/csharp/latest/classGoogle_1_1Apis_1_1Gmail_1_1v1_1_1Data_1_1Filter.html`

The Ruby API does not (yet) support that:

`https://googleapis.dev/ruby/google-apis-gmail_v1/v0.2.0/Google/Apis/GmailV1/Filter.html`

The problem is that maintaining external filters and syncing them to GMail is very complicated and/or consumes a lot of API. Having an eTag would make that very much easier to change only the data which needs to be changed.

Therefore I'd be grateful if the Ruby SDK would support the full GmailV1 API in that regard.

Start with the Ruby Google::Apis::GmailV1::Filter model documented at googleapis.dev and compare it with the Gmail Filter API documentation linked in the issue. Determine how ETags are represented for filters and update the Ruby library so the GmailV1 API exposes that support; done means filter synchronization can use ETags without changing unrelated data.

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