Implement DataStores v2

[nomer888]

This PR implements DataStores v2 APIs. Resolves #22
Some tests are disabled in this branch as they were failing when I forked.

Changes:

• Implemented metadata API to MockDataStoreService:GetAsync and MockDataStoreService:SetAsync with corresponding tests
• Added hack to Utils.importPairsFromTable to inject metadata
• Added hack to MockDataStoreService:ExportToJSON to inject metadata

[nomer888]

I'm currently stuck on finding a good way to make this change compatible with JSON exporting/importing. Currently I'm injecting metadata to exported JSON as the __metadata key under __data, and removing it on import if it's present. I'm not sure there's a non-breaking way to introduce this.

Maybe it'd be better to return a tuple from ExportToJSON; encodedData, encodedMetadata? There's also the question of how versioning from v2 fits into this which I'm not sure how to answer yet.

[buildthomas]

Interesting, are all these tests failing because of the added changes, or were they failing before?

[nomer888]

They were failing before making any changes. I might not have set up my fork correctly (maybe I messed up something w/submodules?)

[NobleDraconian]

I just did a fresh clone of the repository on my end, submodule initialized properly & everything, and the tests failed for me as well.