feat: enhance EnrollmentService.fetchEnrollmentRequests() to filter enrollment requests

[srieteja]

- What I did

• introduce new param 'enrollmentStatusFilter' in EnrollListRequestParam class
• use the above param in EnrollmentService.fetchEnrollmentRequests() to build the command that lists only enrollment requests that match the filter.

- How I did it

• EnrollListRequestParam.enrollmentStatusFilter will be used by users to pass the desired enrollment statuses they wish to list.
• EnrollmentService.fetchEnrollmentRequests() will use this filter to construct a relevant command which will be passed to the server.

- How to verify it

• unit test added, that verifies that appropriate command is generated using the EnrollVerbBuilder and the parsing of response from the server.
• Yet to evaluate if a functional test is needed to validate the change

- Description for the changelog

feat: enhance EnrollmentService.fetchEnrollmentRequests() to filter enrollment requests

[srieteja]

Making this a draft PR until changes in https://github.com/atsign-foundation/at_libraries/pull/530 and https://github.com/atsign-foundation/at_libraries/pull/536 have been consumed by AtServer and that version of AtServer has been promoted to Prod. This measure is being taken to prevent any backwards compatibility issues.