omni-node: add offchain worker (#7479)

# Description

Copy pasted the `parachain-template-node` offchain worker setup to
omni-node-lib for both aura and manual seal nodes.

Closes #7447 

## Integration

Enabled offchain workers for both `polkadot-omni-node` and
`polkadot-parachain` nodes. This would allow executing offchain logic in
the runtime and considering it on the node side.

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Signed-off-by: Iulian Barbu <iulian.barbu@parity.io>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>