snippet gen feature request: add UpdateMask to update samples

Many of the `update` RPC methods require a `FieldMask` in order to have an effect. This is conveyed in the samples like [VideoStitcherServiceClient::updateCdnKey](https://cloud.google.com/php/docs/reference/cloud-video-stitcher/latest/V1.VideoStitcherServiceClient#_Google_Cloud_Video_Stitcher_V1_VideoStitcherServiceClient__updateCdnKey__) where the parameter is required, but in other RPCs such as [LivestreamServiceClient::updateInput](https://cloud.google.com/php/docs/reference/cloud-video-live-stream/latest/V1.LivestreamServiceClient#_Google_Cloud_Video_LiveStream_V1_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)