Annotate types, generated code with @MainActor

[vsanthanam]

Proof of concept to allow UIKit dependencies to exist in the DI graph with with Swift Concurrency

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[wbond]

It looks like to do this we'll need to drop support for Swift < 5.5.

I'm rather inclined to drop support for Swift 5.2-5.4 instead of trying to support them via compiler directives.

There will be some updates in Generator/Package.swift and .github/workflows/ci.yml. There are likely going to be updates to the CI in #416, so perhaps wait until that lands.

[tinder-igornovik]

@vsanthanam is there any way we can merge it? Tinder iOS team is migrating some stuff to support Swift Concurrency and looks like we need this changes. Would you be able to merge them in? Thanks.

[tinder-cfuller]

See #472