feat: support the querystring parameter location and query operation

[AdamFiser]

OpenAPI 3.2.0 added two constructs this library actively mishandles:

Parameter::fromArray(['name' => 'query', 'in' => 'querystring', ...])
=> InvalidArgumentException: Invalid value "querystring" for attribute "in" given.

PathItem::fromArray(['get' => [...], 'query' => [...]])->toArray()
=> {"get":{...}}          // the query operation vanished without a word

• in: querystring treats the whole query string as a single value. It was rejected because the location was not in Parameter::INS.
• The query operation was dropped: setOperation() returns early for methods outside its allow list.

Adds Parameter::IN_QUERYSTRING and PathItem::OPERATION_QUERY. Documents that do not use them are unaffected.

A querystring parameter describes itself with content rather than schema, which #13 made possible.

Tests: ParameterTest::testQuerystringLocation and a new PathItemTest; the existing testInvalidIn message now lists the new location.

[AdamFiser]

Rebased onto current master now that #13 is merged - the duplicated commit is gone and the conflict is resolved. The diff is down to what this pull request is actually about: Parameter::IN_QUERYSTRING, PathItem::OPERATION_QUERY and their tests.

vendor/bin/tester tests -C and vendor/bin/phpstan analyse are both green on the rebased branch.