Add query parameter support to list_items method

[pjpires]

The list collection items endpoint supports query parameters, which can be useful when searching for a specific item in a collection by a field other than its Webflow ID.

This PR introduces a query_params: argument, allowing queries like:

my_item = CLIENT.list_items(@collection_id, query_params: { external_id: 'my-item-external-id' })

Note: Since offset and limit are also query parameters and explicitly included in the method signature, they take precedence over any values in the query_params: hash.

[vfonic]

Thanks for the great PR! I've cherry-picked your changes and they have been merged and released on v1.2.0. Thanks again!