Introduce ExternalSecretBackend CRDs to configure multiple backends

[riccardomc]

Extract Backend instantiation by introducing an ExternalSecretBackend CRD and relative controller.

Backends will be instantiated by creating ExternalSecretBackend CR similar to the following:

apiVersion: externalsecret-operator.container-solutions.com/v1alpha1
kind: ExternalSecretBackend
metadata:
  name: asm-example
spec:
  Type: asm
  Parameters:
    accessKeyID: AKIA...
    secretAccessKey: KSecret444
    region: eu-west-1

ExternalSecret CR will reference the ExternalSecretName to specify which backend to use. For example, the following CR would use the Backend defined above:

apiVersion: externalsecret-operator.container-solutions.com/v1alpha1
kind: ExternalSecret
metadata:
  name: example-externalsecret-asm
spec:
  Key: example-externalsecret-key
  Backend: asm-example

Backend types are now required to register themselves at startup in a:

var BackendFunctions map[string]func() BackendIface

which maps the backend type label to a function that returns a backend of that type.

This will allow the controller to instantiate backends of the correct type when reconciling the ExternalSecretBackend CRs. Instantiated backend are stored in a map as well:

var BackendInstances map[string]BackendIface

An easy way to register backend types at startup is to use the init() function in the module that defines the backend. For example, this is how it is done in the AWS Secret Manager backend:

func init() {
	BackendRegister("asm", NewAWSSecretsManagerBackend)
}

[riccardomc]

Closed because we merged #18 instead.