Implicit attach in `subscribe` and `subscribeAll`

[sacOO7]

• Currently, subscribe and subscribeAll methods in Go automatically initiate ATTACH, which goes against the spec.
• As per latest spec (TB4, RTL7g, RTL7h, RTP6d, RTP6e), implicit attach is enabled only when channelOptions.attachOnSubscribe is set to true, check ably-java#channel.subscribe

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