[SDK-3937] Create a tree-shakable module for realtime publishing

[lawrence-forooghian]

@owenpearson mentioned that we have many browser use cases which only require subscriptions, and no publishing. He suggested that we create a separate tree-shakable module for this functionality.

This commit introduces the API, but the bundle size savings are minimal since it only pulls out the very low-hanging fruit. I think that we could return to this at some point to see what further size savings we could achieve, but I didn’t want to spend too much time on this now.

Resolves #1491.

[lawrence-forooghian]

Yeah, I think you're right that there's more that can be done. I think the mistake that I made is that I looked at the code and thought "oh, well, even if you're not publishing, you still have to send messages like ATTACH, so most of this stuff to do with sending messages can't be removed", but you're right, ACK and all that stuff only applies to publishing and presence, so I'll take a fresh look.

[lawrence-forooghian]

@owenpearson I've pushed another commit which moves the ACK handling into a separate module, which is used by the RealtimePublishing and RealtimePresence modules. The size savings are still tiny, though; it takes BaseRealtime from 102.88KiB on the integration branch down to 100.59KiB.

There is still a bit of msgSerial-related code hanging around in ConnectionManager and MessageQueue, but I think the savings that we'd get from removing it would be even more marginal.

What do you think?

[owenpearson]

@lawrence-forooghian yeah i'm not really sure if it's worth it tbh - would probably be more convenient for the base client to include publishing than to potential save ~2kb for use cases which don't need realtime publishing :/

[lawrence-forooghian]

@owenpearson should I go ahead and close the issue then?

[owenpearson]

@lawrence-forooghian yes please 👍