feat: add support for appset plugin generators

[blakepettersson]

This has one caveat, which likely cannot be fixed until #615 is merged; we're limited to simple string maps for parameters, which is only a subset of what the plugin generator supports. In reality pretty much anything can go in as a parameter from the perspective of the plugin generator, but terraform-plugin-sdk needs an explicit type.

With terraform-plugin-framework we can use any as a type, then we can handle the marshaling/unmarshaling appropriately. I don't believe this will be a breaking change in the future since we will be widening the types parameters will accept at a later stage.

Fixes #620

[khalilj-appsflyer]

Dear maintainers @oboukili @onematchfox @the-technat @mkilchhofer
Can you please review this PR as it fixes a blocker issue for us at AppsFlyer?
Appreciate you kind help.

[the-technat]

LGTM

[the-technat]

@blakepettersson have you also done a manual test locally to see if the fields are set correctly?

[blakepettersson]

@the-technat yes, this TF

resource "argocd_application_set" "plugin" {
	metadata {
		name = "plugin"
	}
	spec {
		generator {
			plugin {
				config_map_ref = "plugin"
				input {
					parameters = {
						key1 = "value1"
					}
				}
				requeue_after_seconds = 30
			}
		}
		template {
			metadata {
				name = "{{cluster}}-guestbook"
			}
			spec {
				project = "default"
				source {
					repo_url        = "https://github.com/argoproj/argo-cd.git"
					target_revision = "HEAD"
					path            = "applicationset/examples/list-generator/guestbook/{{cluster}}"
				}
				destination {
					server    = "{{url}}"
					namespace = "guestbook"
				}
			}
		}
	}
}

Will create this appset

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: plugin
  namespace: argocd
spec:
  generators:
  - plugin:
      configMapRef:
        name: plugin
      input:
        parameters:
          key1: value1
      requeueAfterSeconds: 30
      template:
        metadata: {}
        spec:
          destination: {}
          project: ""
  template:
    metadata:
      name: '{{cluster}}-guestbook'
    spec:
      destination:
        namespace: guestbook
        server: '{{url}}'
      project: default
      revisionHistoryLimit: 10
      source:
        path: applicationset/examples/list-generator/guestbook/{{cluster}}
        repoURL: https://github.com/argoproj/argo-cd.git
        targetRevision: HEAD
  templatePatch: ""