Deliver multiple KeyVault support

[mnriem]

Summary

Delivers support for multiple keyvaults

Issue Type

• New Feature

Starter Names

• key vault spring boot starter

Additional Information

[saragluna]

1. We'd better also change the document to demonstrate how to use multiple KeyVault, and clarify the precedence of them when containing the same key.
2. I'm not sure which pattern is better to organize the KeyVault properties:

• The pattern used in this PR, <keyvault-name>.azure.keyvault.xxx, will keep the implementation neat.
• The pattern like this azure.keyvault.<keyvault-name>.xxx will make the properties organized in one unified namespace.

[mnriem]

When you say the document which one do you mean?

On the order yes we need to clarify that the order of the .names attribute determines the order in which the respective KeyVaults are consulted.

On the naming convention I opted for the key vault name at the beginning as it was easier/cleaner in the implementation to add it, but I have no preference. So if it needs to be changed just say the word.

[saragluna]

Sorry for not mentioning where to add them. A section describing the new feature added to the README.md will be enough.

If the <keyvault-name> has no usage other than distinguishing different configurations, we'd better use array to list these configurations, in which case the order could be expressed by their order showing up in the array.

If we want to use the <keyvault-name> pattern, I think azure.keyvault.<keyvault-name>.xxx is better even though the implementation isn't clean/compact.

[mnriem]

OK I will change it to use to the azure.keyvault.<keyvault-name>.xxx syntax. I will rename the .names property to .order as it expresses what it is used for from the customers perspective, it is required however as it is also used to determine whether or not to enable multiple key vault support or not.

[saragluna]

There're some unit tests failed.

[saragluna]

What if the user also sets the azure.keyvault.uri, what's the order between them?

[mnriem]

The order would be dependent on the order in the file, but I feel we should caution a customer against doing that. If they opt for multiple key vaults they should only use it that way so the order is guaranteed. I will add this to the documentation.