[backport#17316] refactor: Replace all uses of boost::optional with our own Optional type

Summary:
refactor: Replace all uses of boost::optional with our own Optional type (Wladimir J. van der Laan)

Pull request description:

Replace all uses of boost::optional with our own Optional type. Luckily, there aren't so many.

After this:

- `boost::optional` is no longer used directly (only through `Optional` which is an alias for it)
- `boost/optional.hpp` is only included in one place

Backport of Core PR17316

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7388