Handle new `Migrate` action and ACK type

[michalpristas]

DoD

• OpenAPI spec is updated to include MIGRATE action
• Fleet server recognizes new MIGRATE action (definition below)
• Fleet server can ACK MIGRATE action
• Tests are up to date with new action set

Action definition

MigrateAction:
 type: object
 properties:
   id:
     type: string
     description: Unique identifier for the action.
   type:
     type: string
     description: Type of the action.
   target_uri:
     type: string
     description: URI of Fleet Server in a target cluster
   enrollment_token:
     type: string
     description: Enrollment token used to enroll agent to a new cluster
   settings:
     description: |
      An embedded JSON object that holds user-provided settings like TLS.
      Defined in fleet-server as a `json.RawMessage`.
     type: string
     format: application/json
     x-go-type: json.RawMessage
 required:
   - id
   - type
   - policy_id

Supported keys for settings are (more details here):

• ca-sha256
• certificate-authorities
• elastic-agent-cert
• elastic-agent-cert-key
• elastic-agent-cert-key-passphrase
• header
• insecure
• proxy-disabled
• proxy-header
• proxy-url
• staging
• tag
• replace-token

follow camelCase for naming properties in settings json object