instance: Add OCI container support

[maveonair]

This pull requests adds an initial support to create a OCI container with this provider.

Example

resource "incus_instance" "redis" {
  project = "default"
  name    = "redis"
  image   = "docker:redis"
}

What's been done

• Added support for OCI protocol
• Added environment. to the computed keys list in resource_instance.go
• Added test to run an OCI image
• Added DockerHub as remote for test environment

[maveonair]

@stgraber I'm assuming we need to add DockerHub as a new remote in the test environment(s) to run a test to create an OCI container. According to https://github.com/lxc/terraform-provider-incus/blob/main/.github/workflows/test.yml#L62:

...
incus remote add docker https://docker.io --protocol=oci
...

[stgraber]

Yep!