[FEATURE] Snapshot action validation

[Angie-Zhang]

Clarify Snapshot action validation conditions and implement it.

Meta Feature https://github.com/opensearch-project/index-management/issues/587

Validation Conditions Most errors of snapshot action come from the configuration and connections of the snapshot repo. i.e. repo is in snapshot deny list; connection of repo is broken; repo is missing, etc.. We will consider repo validation and dry-run during policy creation instead of before action execution. Meanwhile, there is individual snapshot management function since 2.1, we should consider to avoid snapshot action conflicts with snapshot management at policy level. At current stage, Error Prevention only validates index existence and name.