Add AblyConnector and associated Channel classes

[owenpearson]

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Note: this PR is one of four parallel pull requests to add first class support for Ably to Laravel. Please find the other PRs here:

• laravel/docs#8120
• laravel/framework#43685
• laravel/laravel#5969

Background: the current Ably broadcaster uses the Pusher JavaScript SDK and the documentation instructs users to use Ably with the Pusher compatibility mode enabled. We’ve received a lot of feedback that various features of the Ably Laravel integration don’t work as expected, often caused by the fact that there’s a dependency on the Pusher JavaScript SDK which we have no control over. This collection of PRs adds a new broadcaster which uses the ably-js SDK which means that we can ensure that the Ably broadcaster will remain stable. We are more than happy to take feedback and answer any questions about the work presented here :)

• Adds a new AblyConnector and associated Channel classes to the library which are compatible with the Ably JavaScript SDK.
• Also adds some integration tests to ensure the new connector continues to work as expected.

See laravel/docs#8120 for usage instructions.