chore: Add feature flag for new SCIP-based GraphQL APIs

[varungandhi-src]

Makes partial progress towards https://linear.app/sourcegraph/issue/GRAPH-721

After this, I'll make necessary changes to the various configs to enable this feature flag
for dev, S2 and Sourcegraph.com. After that, I'll change the default to be false.

Test plan

• Manually test that using experimentalFeatures.scipBasedAPIs can turn the feature on/off

Changelog

[kritzcreek]

(Need to run sg bazel configure)

When turning the feature off via site-config I'm getting a not super pretty error, but I don't think we care about that. Can confirm it disables the API

{
  "errors": [
    {
      "message": "graphql: got nil for non-null \"UsageConnection\"",
      "path": [
        "usagesForSymbol"
      ]
    }
  ],
  "data": null
}