rpc: Replace boost::variant with std::variant for RPCArg.m_fallback

Summary:
This is a backport of core#20736

Depends on D12797

Test Plan:
With UBSAN
ninja && ninja check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12798