Crypto: favor direct support for key vaults over external secret storage

[reinkrul]

About a year ago, the concept of "external secret storage" was introduced to let parties use key storage other than HashiCorp Vault by implementing an API. As far as I know, this was to cater Azure Key Vault users since. At the time, we thought you needed the quite bloated Azure SDK completely, and we didn't want/couldn't implement/test the feature (since no core maintainer was actually using Azure).

Things now changed:

• People are that use Azure Key Vault want to contribute this feature (me)
• Using the Azure SDK for a specific service (Key Vault) only requires you to depend on azcore and azkeys, which aren't that big dependencies at all
• The external secret store API in still marked "experimental" since it's missing feature (e.g. security), which nobody seems willing to contribute. So it's another unmaintained feature.

Suggestion:

• Drop support for secret store API (or at least deprecate it) in favor of directly supporting stores (HashiCorp Vault and Azure Key Vault).
• Find out impact of people having to migrate from using HashiCorp Vault through Secret Store API to directly using it from the Nuts node (just configuration, or does it require migration?)