Metadata toMergedBuilder is missing code to handle appMetaData

[akafredperry]

Describe the bug

The field appMetadata has recently been added to org.atsign.client.api.Metadata but there is code and (tests) missing for toMergedBuilder and merge since they have not be amended to include this new field.

Steps to reproduce

1. Create a test that builds some Metadata with appMetadata
2. Invoke toMergedBuilder().build()
3. The appMetadata is "lost"

Expected behavior

New tests which verify that appMetadata is preserved / resolved correctly should pass

Screenshots

No response

Smartphones

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Were you using an atApplication when the bug was found?

No response

Additional context

No response