Allow (specified) additional keys in plugin validator

[nataliecarey]

What

We're experimenting with adding keys to the config to support Frontend v5. The addition of these keys breaks the validation, in a lot of cases we'll be able to publish updates to the validator which resolve this but there are some cases where it's too early to do that. In these cases we should allow the plugin developer to specify which keys they want ignored by the validator e.g.

npx govuk-prototype-kit@latest validate-plugin --keysToIgnoreIfUnknown="oneNewKey,anotherNewKey"

This will help any time we're in the early stages of adding new features to the interface. The behaviour should be:

• When oneNewKey (from the example above) isn't known to the validator it should be accepted
• When oneNewKey (from the example above) is known to the validator it should be validated as usual

Why

So that we can more easily experiment with interface design before committing to it.

Who needs to work on this

• Developer
• Content Designer (for the interface keysToIgnoreIfUnknown)

Who needs to review this

• Developer
• Content Designer (for the interface)

Done when

• New keys can be specified by the person running the validator
• These new keys are ignored if they aren't known to the validator
• These new keys are validated as usual if they are known to the validator

[nataliecarey]

Relates to #2293 (and similar tickets in the future)