Implement PATCH behavior for user edits

[bushidocodes]

When we edit users, we want to be able to do partial edits and not wipe out fields inadvertantly.

gRPC3 assumes PUT or full edits, as they serialize default values for any fields in a message that aren't explicitly set (edited). You can read about this here: https://developers.google.com/protocol-buffers/docs/proto3#default

This makes it harder for us to be explicit with what we want to set. We could filter out default values when updating data, but that's not great, as sometimes, we actually want to set a value to 0 in an edit.
The solution to this a gRPC FieldMask, which is a string that expresses which fields we want to set
https://developers.google.com/protocol-buffers/docs/reference/csharp/class/google/protobuf/well-known-types/field-mask

So to achieve this:

1. The relevant protobuf messages need to add a fieldmask
2. The client needs to set the fieldmask
3. The correct node needs to read the fieldmask and only update the fields contained inside