Add Base.xcconfig and push most code signing settings there

[mokagio]

Fix

This was prompted by some changes in the app identifiers (plural because we have one for release and one for debug) that required regenerating the provisioning profiles.

Internal ref pdnsEh-1Kz-p2

Adds a Base.xcconfig file and moves code signing settings there, so they are DRY and we don't need to deal with Xcode adding many lines to the project file every time we tweak them.

The only setting that stayed in the project itself (i.e. we'd need to use the Build Settings GUI to modify it) is PROVISIONING_PROFILE_SPECIFIER because this is a per-target setting and we don't have that level of granularity in the xcconfigs

Test

You should be able to run a development build successfully after having downloaded a new development bundle from our Mission Control (Automatticians only).

I run a build locally in release mode to verify that mode builds, too.

Review

Only one developer or infra engineer required to review these changes, but anyone can perform the review.

Release

These changes do not require release notes.

[AliSoftware]

Reviewed the code changes / diff and LGTM, thought disclaimer that I haven't tested it in Xcode; I'll let a SN dev check it works as expected with their setup (eg even if they don't have release certs or ASC account in their Xcode like we AppsInfra might have)

The only setting that stayed in the project itself […] is PROVISIONING_PROFILE_SPECIFIER because this is a per-target setting and we don't have that level of granularity in the xcconfigs

Could be worth adding a // comment in the .xcconfig to mention that? (until we have time to create per-target xcconfig files in the future to solve that properly 😛)