[openfeature] add close() to the adapter

[dferber90]

Follow-up to #474, which corrected the initialization retry behavior and closed #473. This PR contains only the shutdown mechanism.

The condition

The adapter had no shutdown mechanism. Section 2.5 of the OpenFeature provider specification gives requirements for such a mechanism.

The new close() function

• close() puts the adapter in its uninitialized state. The next evaluation initializes the adapter again. See requirement 2.5.2.
• If an initialization is in progress, close() first waits for the initialization. The client then goes to onClose, and the adapter does not leak the client. If an evaluation starts during the shutdown, the evaluation waits, then it initializes the adapter again. See requirement 2.5.2.
• close() is idempotent. Concurrent calls share one shutdown. A second call does nothing more, if no initialization occurred between the two calls. See requirement 2.5.3.
• If the initialization failed, there is nothing to dispose of. close() then does not call onClose.

Requirement 2.5.1 is about the disposal of resources. The new onClose option does this, because the adapter cannot do it correctly:

createOpenFeatureAdapter(init, { onClose: () => OpenFeature.close() });

An OpenFeature client has no function to close itself. To shut down a provider, you must close the provider on the global OpenFeature API. But that also closes providers that this adapter did not register. Thus the user must give permission for the disposal.

The new options parameter is an addition to both overloads. Existing code continues to operate.

Be careful: if you give a Client to the adapter, and not an init function, the adapter has nothing to make again. Evaluations after close() then continue to use the same client. The README gives this information.

Tests

There are 7 new tests for close(). I also did mutation tests on the two most difficult lines: the guard for a shutdown in progress in initialize(), and the wait for the initialization in progress. If you remove one of these two lines, one test fails.

All 21 tests, pnpm type-check, pnpm check, and pnpm build are successful.

Specification

https://openfeature.dev/specification/sections/providers#25-shutdown

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