Add krel validate cmd

[npolshakova]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Introduces new krel validate <release-notes-maps> command for release notes team to use when checking for invalid formatting.

The release notes team is blocked whenever invalid yaml is merged into sig-release:

kubernetes/sig-release#2589
kubernetes/sig-release#2541

Invalid yaml can be introduced during the review process when people apply changes or manually edit the maps (not through krel, but in an editor).

Examples of runs:

Valid check:

krel validate sig-release/releases/release-1.30/release-notes/maps
...
Validating YAML file: sig-release/releases/release-1.30/release-notes/maps/pr-123935-map.yaml
File sig-release/releases/release-1.30/release-notes/maps/pr-123935-map.yaml is valid YAML.
YAML file sig-release/releases/release-1.30/release-notes/maps/pr-123935-map.yaml is valid.
Validating YAML file: sig-release/releases/release-1.30/release-notes/maps/pr-124001-map.yaml
File sig-release/releases/release-1.30/release-notes/maps/pr-124001-map.yaml is valid YAML.
YAML file sig-release/releases/release-1.30/release-notes/maps/pr-124001-map.yaml is valid.
All release notes are valid.

Invalid yaml:

Validating YAML file: sig-release/releases/release-1.30/release-notes/maps/pr-112957-map.yaml
FATA failed to validate release notes: YAML validation failed for sig-release/releases/release-1.30/release-notes/maps/pr-112957-map.yaml: YAML unmarshal failed for sig-release/releases/release-1.30/release-notes/maps/pr-112957-map.yaml:error converting YAML to JSON: yaml: line 3: found character that cannot start any token 

Which issue(s) this PR fixes:

Catches occurrences of #2753

Special notes for your reviewer:

Eventually, this command can be added to release-actions and run as a workflow similar to kubernetes/sig-release#2547.

Does this PR introduce a user-facing change?

Introduced new `krel validate` command for validating committed release note edits. 

[npolshakova]

@satyampsoni this can be used to validate the yamls instead of relying on the script. We can move this to a GitHub action once it goes in.

[k8s-ci-robot]

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This pull-request has been approved by: npolshakova
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