EAM: AppId defined as UUID, but GSMA EWBI defines it as a string

[gainsley]

Problem description
There is some inconsistency between the definition of an AppId here in CAMARA NBI, and AppId in GSMA EWBI. Here is it a UUID, but in EWBI it is a string, specifically ^[A-Za-z][A-Za-z0-9_]{7,63}$.

Expected behavior
Identifiers for the same thing should be consistent between NBI and EWBI, otherwise we'll get into messy conversions back and forth.

Alternative solution

Additional context
This needs further discussion on which approach to use, and how we can coordinate keeping these two APIs in sync given their different governing bodies.