Store preferred border agent extended address for each thread dataset

[emontnemery]

Breaking change

To add a Thread credentials to iOS and Android a preferred Thread border router needs to be defined. With this update, the preferred border selection will be lost. You'll have to set a preferred border router again before being able to add Thread credentials to a phone.

Proposed change

Store preferred border agent extended address for each thread dataset

This is needed by iOS

Type of change

• Dependency upgrade
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• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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• This PR fixes or closes issue: fixes #
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