Revert client instantiate inside of init method. Create with empty …

[leoromanovsky]

…in-memory store. Populate the configuration store in init.

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labels: mergeable

Fixes: #issue

Motivation and Context

Client instantiation was pushed from a static method to the init function (b76e8d8)

This means that accessing the instance using EppoClient.getInstance before calling init provided an undefined variable. Making any API calls against it led to crashes.

Description

• Restores client instantiation in the static method so that it is immediately available. We need to select some kind of configuration store - previously this defaulted to localStorage. Now I am changing this to load from an empty memory store. This changes the client behavior so that between getting the client and calling init it will always evaluate to null.
• The init method sets the desired configuration store or the factory method detects which environment it is in.

How has this been tested?