Support root-level webhooks for OpenAPI 3.1 documents (parse + WebhooksIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adds a root-level webhooks field:
a map of names to Path Item objects describing requests the API sends rather than receives.
3.0 has no equivalent field.

This PR adds parse support and version-mismatch detection.

Parse layer

webhooks is parsed as a hash of PathItem objects (with $ref support),
exposed as an accessor on the root object:

root = OpenAPIParser.load('spec_3_1.yaml')
root.webhooks['invoicePaid'] # => OpenAPIParser::Schemas::PathItem

Following the permissive-parse strategy agreed in #152,
the parse layer accepts webhooks regardless of the declared OpenAPI version.

SpecValidator rule

WebhooksIn30 reports a violation when a 3.0 document declares root-level webhooks:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [WebhooksIn30] #/webhooks — `webhooks` is a 3.1 root-level addition; 3.0 documents have no such field

[geemus]

lgtm, thanks!