Add getJSON method

[jaxx2104]

Context:

• There is a method called jsonValue in the iOS and Android SDKs. (FIRConfigValue)
• The method is not available on the Web, but the Firebase console has a JSON Editor, so it would be nice to have a method to get JSON.

This PR did the following:

• Add getJSON method ✍️
• Add unit test ✅

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[jaxx2104]

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