Add Notification Registry for ODP Setting Updates

[opti-jnguyen]

Summary

(Copied from corresponding Java PR: optimizely/java-sdk#501)

• NotificationRegistry is added to create default NotificationCenter for internal use for updating ODPConfig.
• This is to handle a corner case that if in any case user passes different notificationCenter to Optimizely and ProjectConfigManager then the Notification of UpdateProjectConfig will not trigger because its handler is getting added in the main NotificationCenter object.
• This will make sure UpdateProjectConfig will update ODPConfig every time irrespective of different NotificationCenter passed by user.

Note: cachedConfig doesn't appear to be needed for JS implementation as getConfig() already returns a locally stored configObject: ProjectConfig in memory without the need to await for a return

Test plan

• Added additional unit test to test NotificationRegistry
• All tests should pass.

Issues

• FSSDK-8779

[mikechu-optimizely]

Just some thoughts. I'm not sure I'm the official code reviewer so....

[jaeopt]

A suggestion to clean up redundant sdkKey in 2 places.

[jaeopt]

We have sdkKey here (not indirectly from projectConfigManager) in line 132.
We can consider remove "getSdkKey()" new method from projectConfigManager and use a local value.
Also, we have to change "sdkKey" as mandatory (not null), which may be a breaking change in the release.

[opti-jnguyen]

Thanks for the catch - swapped out .getConfig()?sdkKey references with local config.sdkKey usage.

Regarding changing sdkKey to be mandatory, this will definitely be a breaking change - especially since there's some nuances (for example, the Lite bundle does not expect sdkKey to begin with). Brainstorming on this one.

[msohailhussain]

have some questions, we may discuss offline.

[msohailhussain]

All this logic should come-up after the readyPromise? How config.sdkKey will be non-null if the datafile is still being fetched.

[opti-jnguyen]

Moving logic into Promise.all.

Also, as discussed, this should catch for both cases where a new Optimizely instance is provided either an SDK Key or manually includes a datafile.

[jaeopt]

A few changes suggested.

[jaeopt]

We have 2 sources of sdkKey, here and projectConfigManager. It looks like sdkKey is optional here (when datafile is provided and no polling required). In projectConfigManager can return a valid sdkKey in either case.
So, it may be a better to get sdkKey consistently from projectConfigManager. In this way, we do not see any breaking changes required.

[jaeopt]

LGTM