Expose metadata-hash feature from polkadot crate (#4886)

Enabling this feature when building the `polkadot ` crate will lead it
to being enabled for the builtin westend and rococo runtimes. The result
of that is that a merkleized metadata hash will be computed (at some
time cost) in those runtimes, which will allow transactions which
include a hash via the `CheckMetadataHash` extension to work.

The idea is that this is useful for being able to test/experiment with
the `CheckMetadataHash` extension against local nodes.

---------

Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <git@kchr.de>