Add finalize() and AutoCloseable support to AblyRest instances

[QuintinWillison]

A dual pronged approach to resolve #801.

Either:

• The addition of finalize() implementation in f29e497 will passively clean up the thread pool executor once GC catches cleans up the AsyncHttpScheduler instance (once it's no longer referenced by the AblyRest instance that created it)
• The adoption of AutoCloseable in 56593e5 will allow app developers to explicitly call close() on their AblyRest instances, or use try-with-resources

[KacperKluka]

LGTM 👍

[ikbalkaya]

I made a comment on a line, otherwise looks good.

[QuintinWillison]

I'm going to move this PR back to Draft state while I work out why the integration tests are failing in the REST suite. It is failing for me locally, possibly as a result of an unrelated move from Java 8 to 11 locally, but that's TBC. Sorry for the noise.

[QuintinWillison]

We've identified that there has been a regression in the sandbox test environment. Specifically, proven using this command:

curl http://sandbox-rest.ably.io/stats -u <key>

Where it currently returns results, instead of exhibit expected behaviour which is to fail with:

Invalid use of Basic authentication over non-TLS transport.

As such, any integration test failures at the moment where they are only from the io.ably.lib.test.rest.RestAuthTest.auth_basic_nontls method are temporary and unrelated to the changes made in this PR.