Consider using all-args constructor automatically when JSpecify is enabled

[sjohnr]

It would be nice to omit the default no-arg constructor and use the all-args constructor when JSpecify annotations are generated (see gh-866). This would allow linting and static analysis tools to consider the generated types fully null-safe with no warnings.

Context:

By default, a no-arg constructor is generated for all Java types. Currently, this constructor is generated as private when generating JSpecify annotations on Java types. The all-args constructor is still optional and the no-arg constructor is used in the generated builder's build() method. Static analysis and linting tools cannot see that the no-arg constructor is private, the builder's build() method validates fields, and the all-args constructor is null-safe. This issue is flagged by IntelliJ code intentions with the following message on fields of the generated type:

@NullMarked fields must be initialized