(feat) Add support for registering custom helper functions

[samuelmale]

Requirements

• This PR has a title that briefly describes the work done including the ticket number. If there is a ticket, make sure your PR title includes a conventional commit label. See existing PR titles for inspiration.
• My work conforms to the OpenMRS 3.0 Styleguide and design documentation.
• My work includes tests or is validated by existing tests.

Summary

Introduces the ability to register custom functions to be used as expression helpers. It also revamps the registry system to rely on the global store defined by the framework.

Screenshots

N/A

[kajambiya]

LGTM