Implement EngagementLauncher logic

[ykyivskyi-gl]

What was solved?
This PR implements EngagementLauncher logic and splits start engagement logic to two phases(setup and coordinator start).

Release notes:

• Feature
• Ignore
• Release notes (Is it clear from the description here?)
• Migration guide (If changes are needed for integrator already using the SDK - what needs to be communicated? Add underneath please)

Additional info:

• Is the feature sufficiently tested? All tests fixed? Necessary unit, acceptance, snapshots added? Check that at least new public classes & methods are covered with unit tests
• Did you add logging beneficial for troubleshooting of customer issues?
• Did you add new logging? We would like the logging between platforms to be similar. Refer to Logging from iOS SDKs → Things to consider for newly added logs in Confluence for more information.

Screenshots:

[igorkravchenko]

Left some comments about unit tests and code comments.

[rasmustautsglia]

Apart from unit tests, looks good

[igorkravchenko]

Looks good, but I think EngagementLauncherTests can be improved by making tests a bit deeper involving property engagement on Glia instance to be asserted instead of local variables, like chatEngagement etc.

[igorkravchenko]

Looks good to me 👍

[ykyivskyi-gl]

!squash