Add External Secret Stores support as an alpha feature

[turkenh]

Description of your changes

This PR adds External Secret Stores support as an alpha feature.

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Create an MSSQL server resource with the following manifest and ensure connection details land in Vault properly:

apiVersion: sql.azure.jet.crossplane.io/v1alpha2
kind: Server
metadata:
  name: example-mssqlserver-hasan
spec:
  forProvider:
    location: "East US"
    version: "12.0"
    administratorLogin: "mssqladminun"
    administratorLoginPasswordSecretRef:
      name: mssql-password
      namespace: crossplane-system
      key: password
    resourceGroupNameRef:
      name: example
    tags:
      provisioner: crossplane
  publishConnectionDetailsTo:
    name: ess-claim-conn
    metadata:
      labels:
        environment: development
        team: backend
    configRef:
      name: vault
  providerConfigRef:
    name: example