Add support for policy groups

[tzarski0]

Adding support for policy group based on configuration group object. Tested in versions 20.12 and 20.15. Few important differences between configuration groups and policy groups are listed below:
gen/definitions/generic/policy_group.yaml:

• added exclude_test: true to model_name: variables (there is no variable value we can test and "go generate" fails when list_value used for test data).
• used "policy_versions" parameter instead of "feature_versions" that is used in configuration group
• added configuration group to test_prerequisites as you cannot assign policy group if configuration group is not deployed

internal/provider/data_source_sdwan_policy_group.go:

• when configuration-group doesn't exists, api returns error "Invalid config group passed". When policy-group doesn't exists, api returns 200 OK with empty payload. Adjusted condition in line 153 to support that.

internal/provider/model_sdwan_policy_group.go:

• added condition in line 279 not to read empty variables list if all variables are empty/optional

internal/provider/resource_sdwan_policy_group.go:

• when configuration-group doesn't exists, api returns error "Invalid config group passed". When policy-group doesn't exists, api returns 200 OK with empty payload. Adjusted condition in line 296 to support that.
• in read function, change the line 307 to always read associated devices