Expose metadata-hash feature from polkadot crate

[jsdw]

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.

[bkchr]

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