Extend functionality of portfolioChildIssuesOf() to improve searchability

Summary

Currently, portfolioChildIssuesOf() only accepts one input parameter in the form of an issue key. This makes searching for child issues of multiple parents unwieldy, and doesn't allow for flexible search of child issues based on other parameters.

Desired Results

Either or both of the below results would increase usability of the function.

1. Allow the function to accept multiple issue keys as a comma separated list. E.g., issuekey in portfolioChildIssuesOf("EPIC-001", "EPIC-002", "INIT-001") returns all child issues of EPIC-001, EPIC-002, and INIT-001.
2. Allow the function to accept a subquery within JQL. E.g., issuekey in portfolioChildIssuesOf(project = ABC and issuetype = Epic and environment = "Desktop") returns child issues of Epics in the ABC project with a specific value in the environment custom field.