feat: add support of custom communication protocols

[raman-kananovich]

the new optional parameter loader is added to referenceResolver function. It
can be used to override the mechanism of loading JSON schemas. For an example it
is suitable for overriding original URIs to load corresponding schemas from
another resources or loading JSON schemas via protocols different from
http/https/fileSystem e.g. pub/sub one.

contains part of implementation of the issue #587

[raman-kananovich]

@BelfordZ, could you please take a look at this PR?

[BelfordZ]

@raman-kananovich Thank you so much for the PR. I really like a lot of the design decisions you made. I hope you don't mind that I've created another PR to implement similar thing in a slightly different way.

If it helps, I could consider cutting a branch-release for this if you have code that is depending on it. Might help you transition over. Otherwise, let me know what you think and if there are some improvements that you require.

[raman-kananovich]

@BelfordZ, thank you for your PR! The new approach you introduced covers all required functionality on our side. And I really like it!

Can I ask you to extend the API of schema-utils-js to give access to the new feature from the upper layer?

For an example:
parseOpenRpcDocument.referenceResolver.protocolHandlerMap['protocol'] = () => {};

[BelfordZ]

on it!

[BelfordZ]

@raman-kananovich json-schema-tools/dereferencer#252