Introduce ContentHandler::importTransform.

ContentHandler::importTransform allows ContentHandler
implementations to apply transformations on page content
upon import. Such transformatiosn may by useful to update
from legacy formats, apply ID rewriting, etc.

Note that the transformation is done on the serialized content.
This allows for a "raw" import implementation that writes
improted blobs directly into a blob store without unserializing
them into an intermediary representation. Implementations may
choose to unserialize, transform, and then re-serialize.

Bug: 65256
Change-Id: I290fdf5589af43def8b3eddb68b5e1c23f6124e8