[resource_datadog_custom_allocation_rules] add the functionality to allow users decide custom allocation rule reorder behaviors

[congx4]

add the functionality to allow users define the custom allocation rule reorder behaviors:

when override_ui_defined_resources is false:

If there are rules in middle of existing rules that are managed by terraform, during plan and apply phase, we error it.
If there are rules at the end of existing rules, during plan and apply phase, we warn about unmanaged rules detected at the end of order.

when override_ui_defined_resources is true:
Whenever they are some UI-defined rules, during plan phase, we warn about UI-defined rules will be deleted. During apply phase, we actually delete any UI-defined rules.
Tested in us5 region, all scenarios are aligned with expected results.