SecretManagement should only allow extension vaults that support multi-registration to multi register

[PaulHigin]

This is related to SecretStore issue (PowerShell/SecretStore#76 (comment)).
If an extension vault does not support multiple registrations but is registered twice in SecretManagement, the vault will be registered twice with different names. This can be confusing to the user and lead to data loss.

Proposed fix is to allow an extension vault to indicate if it can be registered multiple times, and if not then disallow a second registration.

[Viajaz]

Is this on any roadmaps? Quirks like this make this module less palatable in production.