(refactor) make RetryException consructor public

[puncleV]

Make RetryException constructor public to allow using it in the code for easily retry whenever it's needed.

Description

The RetryException which is used for default Failsafe retry handling is now package private which prevents the users from easily introduce retries into their code which have specific cases.

Motivation and Context

Use case: i want to be able to retry specific 400 responses but not all of them, right now i have to define my own failsafe plugin or do some extra steps vs. just importing the RetryException and use it where i see fit.

Example on Kotlin:

 on(HttpStatus.Series.CLIENT_ERROR).dispatch(
              Navigators.status(),
              on(HttpStatus.BAD_REQUEST).call(Problem::class.java) { exception ->
                if (someConditionForBadRequestException
                ) {
                  throw RetryException(exception)
                } else {
                  ...
                }
              },
              anyStatus().call(ProblemRoute.problemHandling())
            )
          )

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.