oc_config_validate: Support set-replace operations

[xavier-contreras]

Currently tests are hardcoded to use set-update logic, eg:

• https://github.com/google/gnxi/blob/master/oc_config_validate/oc_config_validate/testcases/setget.py#L31C30-L31C43
• https://github.com/google/gnxi/blob/master/oc_config_validate/oc_config_validate/testcases/set.py#L24

It would be great if they could support set-replace as well.

This would particularly be useful / important for the init config -- perhaps that can be the first gap to address:

• https://github.com/google/gnxi/blob/master/oc_config_validate/oc_config_validate/runner.py#L228

Vendors do not always implement these functions well so they are good to test, or certain bugs are triggered based on which method is used.