refactor: workflow plugin registration

[Revolyssup]

Description

Fixes # (issue)
Current logic requires the table supported_actions defined in workflow.lua to be changed when any new plugin is added to the workflow plugin. With this change, the registration in the table is offloaded to the newly added plugin by implementing the function workflow_handler()

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[membphis]

do we need to add any new test cases?

[Revolyssup]

do we need to add any new test cases?

This is just a refactor. Functionality is already covered in existing test cases. This is internal logic change. So I dont think test cases are needed