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[bot]]

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

Generated by :no_entry_sign: Danger Swift against 1767a76b09d3d6aa70c757396127468169d42758

[codecov[bot]]

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 :warning:
ElementX/Sources/Application/AppCoordinator.swift	0.00%	5 Missing :warning:
ElementX/Sources/Application/AppSettings.swift	16.66%	5 Missing :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/element-hq/element-x-ios/pull/2976/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/element-hq/element-x-ios/pull/2976/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | `68.49% <5.26%> (-0.26%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq#carryforward-flags-in-the-pull-request-comment) to find out more.

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[github-actions[bot]]

:iphone: Scan the QR code below to install the build for this PR. :lock: This build is for internal testing purpose. Only devices listed in the ad-hoc provisioning profile can install Element Alpha.

If you can't scan the QR code you can install the build via this link: https://i.diawi.com/CKo5tQ

[sonarcloud[bot]]

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