Refactor child communication to own module

[juanscr]

Description

This pull request refactors all the child communication to an isolated module. Although this allows for clear and independent separation for how child proxying works, this is mostly done so in a next PR we can tree shake this module with a feature flag. There is no particular functionality change introduced in this pull request.

Main changes in the PR:

1. Refactor child communication to an isolated module.
2. Added proper unit tests to detail how child proxying and child event relaying works.

Validation

Validation performed:

1. Ran set of tests.
2. Ran child app to see if everything worked as usual.

Unit Tests added:

Yes

End-to-end tests added:

No

Additional Requirements

Change file added:

Yes

Next/remaining steps:

A follow-up PR will make add a feature flag for using this module, which would then make it so all child proxying code is removed in the final bundle if not needed.

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
packages/teams-js/dist/esm/packages/teams-js/src/index.js	180.51 KB (+0.05% 🔺)	3.7 s (+0.05% 🔺)	198 ms (-4.67% 🔽)	3.9 s

[noahdarveau-MSFT]

Just a couple nits and questions to clarify my own understanding, but otherwise looks good to me.