fix: Allow passing additional scopes to getAuthClient in examples helper function

[Dunsin-cyber]

The OAuth scopes in getAuthClient found in examples/oauth/helper were previously hardcoded, making it inflexible. Adding new functionality that required different permissions, such as sending payments, necessitated modifying this core helper function directly.

This change refactors the function to accept an optional additionalScopes array.

Resolves: #488

[rolznz]

Would it be better if all examples pass the scopes they need? because then there isn't the case where they have this scope when they don't need it, or if we want to test a case where the token does not have read permissions

[Dunsin-cyber]

alright.

yea, I wanted to follow that approach too, but I was trying to avoid having to add comments to most examples explaining that "invoice:read" isn’t actually required for the function, it’s just there because at least one permission needs to be passed.