feat: add local override functionality

[yfrancis]

If overrides have been supplied, use those instead of evaluating rules. Leverages a user-supplied configuration store if present, which may be backed by memory or local storage.

[leoromanovsky]

The obfuscation support for this feature would benefit from a design doc with additional eyes on it; it would be a good reason to evolve our security posture for example beyond md5 (a deprecated algorithm).

[aarsilv]

Looks like a good foundation for overrides, and I think getAssignmentDetail() is the right place to check for an override.

We will need to be sure to check in the right place for the precomputed client as well.

I think overrides like this could be useful for testing and simple, client-only deployments. For customers using server flags as well, I think we'll need to on-the-fly adjust targeting rules or something to that effect. A mini design review could be helpful in laying out the various use cases.

Note that if we do release this, we'll want to bump the version in package.json before releasing. This should be developed concurrently with the upstream SDK(s) as well.

[aarsilv]

Upstream overrideStore is not exposed, so I imagine we need a setOverride() method on the client to add an entry to this store, if present.

[greghuels]

Looks good!