Use XcodeGen files for app variants and setup Enterprise submodule.

[pixlwave]

This PR makes the following changes:

Configuration

• Creates an app.yml with the base app variant configuration for XcodeGen.
• Updates the Fastfile to include overrides for this configuration for Nightly/PR builds.
• Adds the app's icon to ElementX/target.yml (technically not necessary but nice to reference the default value here).

Enterprise

• Adds an initial AppHooks mechanism that has an empty implementation but allows for modifying the AppSettings type as a starting point.
• Add a submodule for EXE.
• Conditionally import the Fastfile from that module if cloned.

[github-actions]

	Warnings
⚠️	Some of the commits are missing ticket numbers. Please consider squashing all commits that don't have a tracking number.

Generated by 🚫 Danger Swift against 1767a76

[codecov]

Codecov Report

Attention: Patch coverage is 5.26316% with 18 lines in your changes missing coverage. Please review.

Project coverage is 77.25%. Comparing base (41d40d8) to head (1767a76).
Report is 3 commits behind head on develop.

Files	Patch %	Lines
ElementX/Sources/Hooks/AppHooks.swift	0.00%	8 Missing ⚠️
ElementX/Sources/Application/AppCoordinator.swift	0.00%	5 Missing ⚠️
ElementX/Sources/Application/AppSettings.swift	16.66%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2976      +/-   ##
===========================================
- Coverage    78.05%   77.25%   -0.80%     
===========================================
  Files          693      694       +1     
  Lines        54208    54288      +80     
===========================================
- Hits         42313    41942     -371     
- Misses       11895    12346     +451     

Flag	Coverage Δ
unittests	68.49% <5.26%> (-0.26%)	⬇️

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[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud