snippet gen feature request: add UpdateMask to update samples

[bshaffer]

Many of the update RPC methods require a FieldMask in order to have an effect. This is conveyed in the samples like VideoStitcherServiceClient::updateCdnKey where the parameter is required, but in other RPCs such as LivestreamServiceClient::updateInput, it's not obvious to the user that this is required. And the effect of not knowing this is that the RPC call has no effect (no fields are updated), even though the request is successful.

When the updateMask parameter is marked optional, it would be more useful if we included creating the updateMask as part of the sample.

This may be addressed once we release support in PHP for method_signature annotations (see https://google.aip.dev/134)