Pipeline fails to run remotely

Beam fails to run remote pipeline when the class loader has URLs other than _[file://_](file://_/)._ The environment uses the [Manifold project](http://manifold.systems), which installs a custom URL handler of the form: _manifoldclass://_, which contributes to the URL classpath, however Beam throws an exception when it encounters this URL. Instead Beam should *ignore* or at most warn about URLs it does not recognize.

A similar issue was recently fixed in the Spring framework: [https://github.com/spring-projects/spring-framework/issues/21950](https://github.com/spring-projects/spring-framework/issues/21950). Beam should follow suit.

See external Manifold issue: [https://github.com/manifold-systems/manifold/issues/143](https://github.com/manifold-systems/manifold/issues/143)

Imported from Jira [BEAM-8875](https://issues.apache.org/jira/browse/BEAM-8875). Original Jira may contain additional context.
Reported by: rsmckinney.

Start by locating Beam's remote pipeline classpath handling and the code that rejects non-file URL schemes; compare the expected behavior with the linked Spring Framework issue. Done means a class loader containing a custom URL such as manifoldclass:// no longer causes the remote pipeline to fail, and it is ignored or warned about.

Written by the indexing model from the issue text.