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
Depends on D5979
This is a partial backport of Core PR15638