chore: add full example for s3 log source

[markosfount]

Background

This is a small enhancement as part of "reviving" the Panther terraform provider, in order to be able to better test the provider during development. It is inspired from providers such as the aws terraform provider, who include some "runnable" examples of creating resources using their provider,

Changes

The changes here introduce a more complete example of how the terraform code for creating an S3 resource would look like, as well as instructions of how to run the example with using the provider code in this repo, rather than the published provider code.

This is useful when developing the provider, as one can build the provider and deploy a resource in their dev environment and test and verify how the log source looks and behaves. It is meant to compliment the existing tests.

It can be also useful for customers who clone the repo to see how using the provider would look in a more complete setup, with creating a resource, having input variables, outputs etc.

Testing

This is not changing any code for the provider as it is only an example. It has been tested by following the instructions in the README and running relevant terraform commands to verify that creating the resource:

terraform plan -var-file="test-vars.tfvars"
terraform apply -var-file="test-vars.tfvars"
terraform destroy -var-file="test-vars.tfvars"