Allow users to specify arbitrary tags on entities

[zpatrick]

Allow users to create and filter entities based on tags. Some examples:

l0 environment create --tag owner:zpatrick --tag tier:testing dev
l0 environment get --tag owner:zpatrick
ENVIRONMENT ID    ENVIRONMENT NAME   ....   TAGS
dev123            dev                       owner:zpatrick
                                            tier:testing

Tag would be a new entity command:

l0 tag create --environment dev owner:zpatrick
l0 tag delete --service dev:redis owner:zpatrick

Would also need to support tags in Terraform:

resource "layer0_environment" "foo" {
    name = "foo"

    tag {
        key   = "owner"
        value = "zpatrick"
    }

    tag {
        key   = "tier"
        value = "testing"
    }
}

Keys already used by Layer0 would need to be reserved, e.g. environment_id, version, etc.