fix-810: Updated custom remediation for non-str object types

[emdeefive]

Closes #810

What's Changed

In #810, a client received an exception when the join method was used on a dict custom remediation object.

The fix is simply to verify the object type of the custom remediation before creating the ConfigPlan object.

Manual testing verified that this fix worked with a traditional custom remediation as well as a dict object.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• n/a Attached Screenshots, Payload Example
• n/a Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example App Updates (when adding/changing features)
• n/a Outline Remaining Work, Constraints from Design

[jeffkala]

@itdependsnetworks you're good to merge?