Provide a way to dynamically generate Vault authentication token

[nstuart-idexx]

We are looking to integrate with Vault, and the current VaultConfiguration class does not have a way to generate the authentication token at runtime.

Our use case is that Vault is secured using Googles IAM authentication and service accounts. We need to be able to generate an IAM token to be used for authentication at runtime, and not have it hard coded in a property.

Looking at the current code, I would have to override and replace VaultClientConfiguration and shove our IAM token stuff in there. I think it's possible, but it's not very nice or flexible. Spring provides a simple ClientAuthentication interface you can implement to provide the token, and something similar would suffice.

If I have time I'll see about getting a more thorough proposal together, but wanted to throw this out there and see if maybe my approach has an obvious downsides.

[nstuart-idexx]

Also, this is definitely a feature/improvement request, but doesn't look like I can add that label.

[MaheshAravind]

Is there any update on this?