refactor: Replace std::optional<bilingual_str> with util::Result

Summary:

refactor: Replace std::optional<bilingual_str> with util::Result

https://github.com/bitcoin/bitcoin/pull/25977/commits/8aa8f73adce72e1ae855b43413c1f65504423cb7

Add nodiscard where ignoring a Result return type is an error

https://github.com/bitcoin/bitcoin/pull/27774/commits/dddde27f6fbcff7cdb31f7138efc5d8363537b03

fix includes for touched header files (iwyu)

https://github.com/bitcoin/bitcoin/pull/27774/commits/fa5680b7520c340952239c4e25ebe505d16c7c39

This is a partial backport of core#25977 and core#27774

Test Plan:
ninja all check-all

grep -r 'std::optional<bilingual_str>' src/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14992