Client logging API: methods for users to access the SDK's log messages

[hsubox76]

Adds

• firebase.setLogLevel() - set threshhold log level to show
• firebase.onLog() - add a user specified callback that has access to log level, message, and name of package (e.g. app, firestore, auth) that logged it

[Feiyang1]

Overall it's pretty good. A couple of things:

1. The implementation is different than the proposal where the instance of the service is passed to the user defined callback as a parameter. We should update the proposal with the reasons for the new API and get the approval again.

2. Performance has a special logger to upload performance metrics to the backend.
   Any action on it should not trigger the user defined onLog callback. We will need to introduce a flag or something to tell the special loggers apart and they should not participate in onLog.
   https://github.com/firebase/firebase-js-sdk/blob/master/packages/performance/src/services/perf_logger.ts#L95

I actually think it's a misuse for perf to use @firebase/logger for the purpose of uploading metrics. So another approach can change perf to not use @firebase/logger for uploading metrics.

[hsubox76]

I am making a PR for the perf logger conflict issue: #2460.