new: Support tier in LKE resource and data source

[yec-akamai]

📝 Description

Support tier for LKE-Enterprise in lke & lkeclusters resource and data sources.

✔️ How to Test

make PKG_NAME=linode/lke int-test

make PKG_NAME=linode/lkeclusters int-test

Manual Test:

1. In a sandbox environment, make sure you have set up your account correctly to call the API and put the follow script to create a lke cluster with tier:

provider linode {
    api_version = "v4beta"
}

resource "linode_lke_cluster" "test" {
    label       = "test-lke"
    region      = "us-central"
    k8s_version = "1.31"
    tags        = ["test"]
    tier = "standard"

    pool {
        type  = "g6-standard-1"
        count = 3
        tags  = ["test"]
    }
}

2. plan & apply it. Observe the resource has been created without any issue.
3. Destroy the resource

[ykim-akamai]

LGTM, thanks for updating all the tests!