Provide customActiveChannel prop for ChannelList

[DmitriyNikolenko]

Motivation
React SDK has the ability to set active channels when we navigate to ChannelList which is useful if we wanna highlight a target channel. React Native SDK does not have it as I see.

Proposed solution
Provide 'customActiveChannel' property.

Acceptance Criteria
The user is able to see some channels as active when they navigate to ChatList.

[vishalnarkhede]

hmm we have that in React SDK since channel list and channel components go side to side in the same screen. But thats not the case for RN or mobile.

Curious about the business requirements here, so that we can act on it.