SLM config and retention are optional

[swallez]

Fixes #1917

The config and retention fields in the SLM policy are optional, and also indices in config.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
slm.delete_lifecycle	🟢	4/4	4/4
slm.execute_lifecycle	🟢	4/4	4/4
slm.execute_retention	🟢	4/4	4/4
slm.get_lifecycle	🟢	16/16	16/16
slm.get_stats	🟢	4/4	4/4
slm.get_status	🟢	4/4	4/4
slm.put_lifecycle	🟢	4/4	4/4
slm.start	⚪	Missing test	Missing test
slm.stop	🟢	4/4	4/4

You can validate these APIs yourself by using the make validate target.

[sethmlarson]

Is this another case of Policy/PolicyRead? If so can we split into two and use @OverloadOf?

[swallez]

The issue was reported for a read operation, and I checked the code: parsing and serialization are symmetrical and have the same tests for those optional properties.

[sethmlarson]

LGTM