When the embedder frame (eg. our LMS app) provides context metadata to be saved
with annotations via the `annotationMetadata` config property, save that with
new annotations in the `metadata` field.

We add this information only when an annotation is created, and not on
subsequent edits, because the main use case in the LMS context is to record the
assignment where the annotation was created. If the user edits that annotation
subsequently in another assignment, we don't want it to be "moved" by updating
the `metadata` field.

Part of https://github.com/hypothesis/lms/issues/5698