Remove ably-cocoa submodule

[lawrence-forooghian]

In #6 I pulled ably-cocoa in as as a submodule to make it easier to develop this plugin and the corresponding ably-cocoa functionality together. We need to remove this submodule before v1 and switch back to getting ably-cocoa from SPM. But add some instructions explaining how to use a submodule if needed in the future for iterating on a large feature that requires ably-cocoa support.

┆Issue is synchronized with this Jira Task by Unito

[lawrence-forooghian]

Also when we do this we should, for the pre-release versions of this plugin, pin to a specific version of ably-cocoa so that we can continue to introduce breaking changes in AblyPlugin.

[lawrence-forooghian]

Also when we do this we should, for the pre-release versions of this plugin, pin to a specific version of ably-cocoa so that we can continue to introduce breaking changes in AblyPlugin.

This is no longer the case post #69