Push various code signing related settings to xcconfig

[mokagio]

Relates to the request to support building for Mac, see pdnsEh-1Sc-p2, but does not make it possible yet. This is mostly a preparatory tidy up.

Regression Notes

1. Potential unintended areas of impact – N.A.
2. What I did to test those areas of impact (or what existing automated tests I relied on) – N.A.
3. What automated tests I added (or what prevented me from doing so) – N.A.

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PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes. N.A.
• I have considered adding accessibility improvements for my changes. N.A.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary. N.A.

UI changes testing checklist: Not a UI PR.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23678-163f6ab
	Version	25.4
	Bundle ID	com.jetpack.alpha
	Commit	163f6ab
	App Center Build	jetpack-installable-builds #9874

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23678-163f6ab
	Version	25.4
	Bundle ID	org.wordpress.alpha
	Commit	163f6ab
	App Center Build	WPiOS - One-Offs #10832

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[mokagio]

I decided to create a single "enterprise" file instead of two dedicated "internal" and "alpha" because for the moment the two configuration use the same value. If/when we'll need build configuration-specific build settings, we can add dedicated files for them.