Add optional ResourceGroupName to LTR cmdlet

[pixia]

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[johnpaulkee]

Approved, just some suggestions to use ResourceIdentifier class to parse resource id.

[pixia]

Approved, just some suggestions to use ResourceIdentifier class to parse resource id.

As discussed offline, ResourceIdentifier does not fit my needs. Will stick to the current implementation.

[cormacpayne]

@pixia some of the Sql tests are failing as a part of this PR -- would you mind taking a look?

Comments addressed