Umbrella crate: exclude chain-specific crates (#5173)
Uses custom metadata to exclude chain-specific crates.
The only concern is that devs who want to use chain-specific crates,
still need to select matching versions numbers. Could possibly be
addresses with chain-specific umbrella crates, but currently it should
be possible to use [psvm](https://github.com/paritytech/psvm).
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Signed-off-by: Oliver Tale-Yazdi <[email protected]>