Add Custom configurations and examples for some common resources

[sergenyalcin]

Signed-off-by: Sergen Yalçın yalcinsergen97@gmail.com

Description of your changes

Fixes #15

This PR adds the following resources with their custom configurations and example manifests:

• google_container_cluster
• google_container_node_pool
• google_service_account
• google_monitoring_alert_policy
• google_monitoring_notification_channel
• google_monitoring_uptime_check_config

For monitoring resources, there is not need custom configuration. For cluster, node_pool and service_account resources, some custom configurations were added.

I have:

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

How has this code been tested

• For each added resource, an external resource was provisioned by using the example manifest.
• For each added resource, update and delete operations were tested.

[turkenh]

@sergenyalcin thanks! Just added some comments.
Not a huge issue but in general having separate commits before and after generation would help reviewing.