Introduce ContentHandler::exportTransform()

Introduce ContentHandler::exportTransform()

ContentHandler::exportTransform() allows content handlers to apply
transformations upon export, such as conversion of legacy formats or
filtering of internal data.

Note that the transformation is applied to serialized content, since
the exporter will generally not unserialize the content blob to an
intermediate form before writing it to the dump. Implementations
may choose to unserialy, then transform and re-serialize.

Bug: 65256
Change-Id: Ic55a8bd8bea13041000b176c7c02c7c5ced76f6d