Fix mysql provider config spec.tls kubebuilder validation enum

[alereca]

Description of your changes

Fixes #137

I have:

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

How has this code been tested

Since the current unit tests do not setup a webhook server it kinda hard to add an automatic test for this.
Tested manually with kubectl and a local kind k8s cluster

apiVersion: mysql.sql.crossplane.io/v1alpha1
kind: ProviderConfig
metadata:
  name: default
spec:
  credentials:
    source: MySQLConnectionSecret
    connectionSecretRef:
      namespace: default
      name: db-conn
  # tls one of preferred(default), skip-verify, or true
  tls: true

> kubectl apply -f /home/alejandro/Documents/operators/crossplane/provider-sql/examples/mysql/config.yaml
The ProviderConfig "default2" is invalid: 
* spec.tls: Invalid value: "boolean": spec.tls in body must be of type string: "boolean"
* spec.tls: Unsupported value: true: supported values: "true", "skip-verify", "preferred"

apiVersion: mysql.sql.crossplane.io/v1alpha1
kind: ProviderConfig
metadata:
  name: default
spec:
  credentials:
    source: MySQLConnectionSecret
    connectionSecretRef:
      namespace: default
      name: db-conn
  # tls one of preferred(default), skip-verify, or true
  tls: "true"

kubectl apply -f /home/alejandro/Documents/operators/crossplane/provider-sql/examples/mysql/config.yaml
providerconfig.mysql.sql.crossplane.io/default2 created

[Duologic]

❤️ YAML