Support for versioned secrets

[ramizpolic]

Idea

We would like to have versioned API operations (fetching/querying/updating) on supported providers, as well as secret key version rewrites.

Strategy

All implementations of StoreClient should take into account SecretKey.Version field.
Additionally, RegexpRewrite should be extended to allow specifying which SecretKey field to rewrite, for example:

type RegexpRewrite struct {
	Field string `json:"field"` // make this an enum with values: "Key", "Version"
	Source string `json:"source"`
	Target string `json:"target"`
}

Acceptance criteria

Running the following sync plan will:

1. Sync secret a with version 3
2. Sync all secrets with version 2 for path d/d/d
3. Sync secret a with version 5 into secret a with version 6

plan:
  - secret:
      key: "a"
      version: "3"
  - query:
      path: "d/d/d"
      key:
        regexp: .*
      version:
        regexp: "2"
  - secret:
      key: "a"
      version: "5"
    rewrite:
      - regexp:
          field: "Key"
          source: "a" # key queries are executed on Key only
          target: "b"
      - regexp:
          field: "Version"
          source: ".*#5" # versioned queries are executed on Key#Version
          target: "6"

This needs to be discussed with the rest of the team.
Feel free to suggest a better implementation plan.