`tencentcloud_clb_customized_config`: Allow heredoc strings in `config_content`

[smorimoto]

For now, we still can't use heredoc strings in config content. Honestly, this feels ridiculous. If there is a better language specification, there is no reason not to use it.

Current:

resource "tencentcloud_clb_customized_config" "this" {
  ...
  config_content    = "client_max_body_size 10240M;\r\nproxy_request_buffering off;"
}

Something I propose:

resource "tencentcloud_clb_customized_config" "this" {
  ...
  config_content    = <<-EOT
    client_max_body_size 10240M;
    proxy_request_buffering off;
  EOT
}

Right now, if you try to do this, the first line will corrupt from the second and subsequent apply runs, but it should not!