Add FindAssignable method to User

[zbarahal]

Feature: Adding a new method to the User interface

What this PR does / why we need it:

Adding support for the find assignable users route https://docs.atlassian.com/software/jira/docs/api/REST/1000.1568.0/#api/2/user-findAssignableUsers

Which issue(s) this PR fixes:

No linked issue - can create if that's required.

Special notes for your reviewer:

I attempted to follow the pattern from User.Find by using the option funcs. The other option I considered was using the query library that's being used in Issue.

[elgranjero]

I need this feature too