[rpc] add snake case aliases for transaction methods

Summary:
This is a backport of core#16378 [2/3]
https://github.com/bitcoin/bitcoin/pull/16378/commits/2c2a1445dc9d22c9d729b8301c8b3f54195bcfcf

Depends on D10263

I also added "fee_rate", because I will use it in the next commit instead of conf_target and estimate_mode to set the fee.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10264