Introduced prototype of equalToJson pattern with file source

[ioanngolovko]

Hello.

Sometimes people need to insert big json/xml data into equalToJson/equalToXml patterns. It's hard to do it and maintain in json files. I took the liberty of simplifying this process.

This PR introduces prototype of equalToJson pattern with file source.
It's open to discuss.

As we can see, source field has unpleasant side affects.
We can just check , that trimmed json ends with ".json" substring and then reprocess it.

One more notion - this will work for remote loading only.

I omitted the XML implementation until we decide on the architecture.

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[oleg-nenashev]

This PR is binary compatible, so no urgency to merge it into the 3.0.0 release, Will try to review ASAP

[oleg-nenashev]

Sorry, I dropped the ball on it. Force push does not trigger an update in the PR.
We didn't unify file notations during WireMock 3 release, so I guess it is still actual and could be integrated as is once updated and once the merge conflict is resolved. Fine with you @tomakehurst