[RUM-1963] Add getCurrentSessionId()

[marco-saia-datadog]

What does this PR do?

Exposes native getCurrentSessionId() method introduced with #1810 and #1616.

Notes

Initially, I have added a BoundedBuffer method to handle callbacks with optional strings. I have decided to remove it in the end since, as opposed to the other SDK functions, getCurrentSessionId() does not perform any action on the backend, and it would not make sense to await its result until the initialization is done (it would actually block the initialization from being called).

Therefore, I have decided to expose DdSdkReactNative.wasInitialized in order to perform a simple check on whether the SDK is initialized or not before returning the current session ID.

Additional Notes

In DdRumImplementation.swift I have used:

    @objc
    public func getCurrentSessionId(_ resolve: @escaping (Any?) -> Void, reject: RCTPromiseRejectBlock) -> Void {

instead of

    @objc
    public func getCurrentSessionId(_ resolve: @escaping RCTPromiseResolveBlock, reject: RCTPromiseRejectBlock) -> Void {

As the @objc macro generates broken and malformed bridge header code otherwise.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change