fix(auth, ios): Initialise static dictionary for custom auth domains

[paulw11]

Description

PR #11925 added support for custom auth domains on mobile.

Custom auto domains are stored in a static NSMutableDictionary called customAuthDomains in the FLTFirebaseCorePlugin class, keyed by the app name.

Unfortunately, this static dictionary is never initialized, so custom domains do not take effect.

This PR adds a static initializer to the FLTFirebaseCorePlugin class to initialize this dictionary

Checklist

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• [x ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
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[Lyokone]

Thanks for the fix @paulw11