univalue: Avoid std::string copies

Summary:

This shouldn't matter too much, unless a really large string is pushed into a json struct, but I think it also clarifies the code.

Backport of core#25714.

Depends on D16285.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16289