rpc/rawtransaction.cpp moves to libbitcoin_server since it should not be
accessed by non-node libraries. The utility following utility methods
move to their own unit rpc/rawtransaction_util since they need to be
accessed by non-node libraries:
- `ConstructTransaction`
- `TxInErrorToJSON`
- `SignTransaction`
https://github.com/bitcoin/bitcoin/pull/15638/commits/0509465542d63a5bbe7296f283f44dd491e74f78
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Depends on D5979
This is a partial backport of Core PR15638