Update key name for writing secrets to provide quick pick options

[owenfarrell]

Writing secrets to Vault covers two scenarios:

1. Creating a new secret
2. Updating an existing secret

The current UI leverages a input box - effectively, freeform test. That makes (2) really easy but makes (1) hard since you have to get the secret name exactly right... and case-sensitivity matters.

Converting this to a hybrid input that blends quick pick options (of existing secret names) and a freeform text (for a new secret) would be ideal.

https://github.com/microsoft/vscode/issues/23633#issuecomment-949919193 documents a potential workaround that enables a quickpick input to support a custom entry as well.