runtimevar: support `hashivault` integration

[giautm]

I found the hashivault already supported in secrets but not on the runtimevar. So I openning this issue to request the support.

I'm also happy to work on the PR for this feature, if you guys accept it.

[giautm]

cc @vangent

[vangent]

Go ahead and send a PR. It will probably be a combination of runtimevar/httpvar (for the runtimevar stuff) and secrets/hashivault (for the hashivault-specific stuff). Please try to stay in the style of those.

Thanks!