Update all occurrences of boost::optional's is_initialized or get members.

(comes out of Skunkworks July 2022)

We should prefer using the API that's common between boost::optional and std::optional where we can, and we can in these cases.

• boost::optional has deprecated the is_initialized member, in favor of the std::optional-compatible name has_value. Change all occurrences of is_initialized to has_value.

• boost::optional has a get member, and std::optional does not. The common subset has two choices to get() calls: operator*() and value(). operator*() does exactly what get() does, but has a weird name and may not actually represent the intent of the caller. get has undefined behavior if the object is empty. Change these to value, which is supported by std::optional and throws on empty, making it a safer default choice. get() can be changed to operator*() if the value() behavior is inappropriate.

Future recurrences will be hard to lint on, but easy to clang-tidy on when we can load plugins into clang-tidy. This conversion is done by a clang-tidy plugin in the first place. In the meantime, the clang-tidy plugin can be compiled in.

The plugin: https://github.com/BillyDonahue/mongo_std_optional_migration