Rework RVPS store config

[fitzthum]

The RVPS config currently looks like this

pub struct Config {
    #[serde(default = "default_store_type")]
    pub store_type: String,

    #[serde(default = "default_store_config")]
    pub store_config: Value,
}

This doesn't really match how we've recently been creating configs inside of an enum and have store_config as an opaque JSON value doesn't seem ideal.

We should probably update this to have an enum containing a config, like we've started doing elsewhere in Trustee.

[fitzthum]

I have a pending PR for this.