Add required-without-page_token information to pagination extension

[ahl]

Previously x-dropshot-pagination was simply the value true. Now it's an object with a field required that is an array of strings that tell us what query parameters are required to fetch the first page (i.e. when page_token is not specified).

This also cleans up some stray doc nits.

[davepacheco]

Do I grok correctly that there's no impact on dropshot consumers here (unless they're using the openapi extension)?

[ahl]

Do I grok correctly that there's no impact on dropshot consumers here (unless they're using the openapi extension)?

Yes; that's accurate.