Pass in client

[TimboTambo]

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [ ] I have validated my changes against all supported platform versions
  • Can you provide further info on this?

Related issues

Somewhat related to Issue 56

Describe the solution you've provided

In order to be able to initialise a single instance of the LD client and share it between multiple, independent React trees, we require the ability to pass in the client (or a promise that resolves to the client) into the provider, rather than have it initialise the LD client for us.

This PR adds the client as an optional prop to the provider so that it can use this instance if passed in, otherwise create one.

Describe alternatives you've considered

Using the launchdarkly-js-client-sdk directly, however, the launchdarkly-react-client-sdk is already used extensively so this would require a large change, and the helpers provided by this library are extremely useful.

We could also use our own fork, but it would be great to benefit from other updates to this library.

Additional context

N/A

[TimboTambo]

@yusinto did you have any more thoughts on this PR or are you happy to approve it?

[bwoskow-ld]

This is now available in 2.24.0. Thank you for the contribution!