[Graphite] Add support for user provided labels on Images.

This also adds support for appending things like _Subset to previous
image labels when we make a new images.

Bug: b/308986480
Change-Id: I2c3afcdae87b88689ce7c393a5f825901e74de0c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/855436
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
6 files changed