Added support for sessionStorage for cache.

[elliotcourant]

This adds support for specifying localStorage or sessionStorage when
bootstrapping the LD JS client. This can be used if the user's hash
changes frequently and can cause a build-up of old records in the
client's local storage by instead storing data in the session storage.

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

This adds support for being able to specify sessionStorage when initializing the LD client. We were running into a problem where user's who were frequently signing in and out would build up several different caches for LD in local storage and had to clear them out manually.

So help improve this going forward this adds support for sessionStorage as an option for LD's cache, which will clear out cached items once the session is over rather than letting them build up.

Describe alternatives you've considered

This is the solution that my team and I came up with given the use case. There might be other approaches to solve the problem and I welcome any and all feedback.

Additional context

This merge request also has a dependency on launchdarkly/js-sdk-common#2

[eli-darkly]

Please see my comments on the js-sdk-common PR. This browser-specific part of the code looks fine but I'm still a little uncertain about the overall approach.