Add strict validation mode for AR resource identifier schemes

[nvmkuruc]

Description of Change(s)

The current implementation of AR resource identifiers (URI/IRI) allows any string to be registered as the scheme component. The URI/IRI specification restricts the character set of the scheme component to ASCII alphanumeric characters plus +, ., and -. Consider handling of a scheme which contains : as an example of where an unrestricted character set for scheme can go awry. This change adds validation, warning the user when a scheme is not valid under the specification. By default, the scheme is ignored, but users can set PXR_AR_DISABLE_STRICT_SCHEME_VALIDATION to be more permissive.

Validation could be implemented as a regular expression match, but was done with explicit checks to produce clearer error messages for users. A note is left to output the invalid characters once #2120 has been accepted.

The test_output scheme currently used in some of the testing is an example of an invalid scheme per the URI/IRI specifications. This scheme has been added to a list of invalid test cases and been replaced with test-output.

Fixes Issue(s)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-8372