573 - Check URLs are valid before attempting to perform HTTP request

[markfullmer]

Purpose

Resolves #573

Implementation

Adds a new isValidUrl() helper function that:

• checks the URL using the standard PHP FILTER_VALIDATE_URL
• checks IPs the host resolves to and uses dns_get_record() to check if a record exists
• if dns_get_record() fails, try gethostbyname()
• Filter any found IPs against PHP's reserved and privileged ranges (FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE)

This helper function is added to Extractor::resolveUri(), adjacent to the check for HTTP URLs.

Finally, the existing isHttp() function is updated to use a stricter "allow-list" methodology, rejecting anything that does not match the http or https schema, as suggested in #573.

Test coverage

Test coverage is added in FunctionsTest. All of its scenarios pass. There are other failing tests, but these are unchanged compared to the failing tests prior to this code change.