Provide MetaGraphDef transformer, and call it from SavedModel export.

A MetaGraphDef transformation consists of a GraphDef transformation provided by the Graph Transform Tool, followed by some surgery at the MetaGraphDef level to remove references to any nodes that were removed.

This allows users to request Graph Transform Tool rewrites integrated with Estimator.export_savedmodel().  It also integrates graph freezing interleaved with those rewrites (even though that is not provided by GTT).

A limitation, for now, is that all Variables and many of their associated Save and Restore Ops are retained even if they are unused and strip_unused_nodes is requested (pending further clarity on which ones may be safe to remove).

PiperOrigin-RevId: 163475341