feat: ux improvements for pre-emptive key validation

[kriscoleman]

UAT looks great!

Trying to add a duplicate key

Performing an openfeature command when the manifest has duplicate keys

The only improvement I'd suggest for a later iteration is, when adding a new flag, to either find a way to detect the duplicate key early or allow the user to change the key name without having to start over.

Originally posted by @georgesdugue in #200 (comment)

Description

We can improve UX for our users if we pre-emptively validate keys on manifest add. A further step to give the user a chance to attempt a new key name without starting over will reduce frustrations and provide for seamless use.

Requirements

1. when using manifest add to introduce a new flag key, the cli will pre-emptively check the key validity before moving on to prompting the user for the rest of the flag details.
2. when a duplicate key is detected by the pre-emptive validity check, the cli will give the user a chance to retype their key with a new value without needing to start over.