Pls add a dmr handler to add and remove a single environment variable

For adding a single jvm-option, operations exist for convenience:
add-jvm-option
remove-jvm-option

The same would be good for environment variables:
add-environment-variable
remove-environment-variable

At the moment, the same can only be achieved by writing the whole attribute environment-variables, which does not allow to add a single variable (if some already exist, the existing ones have to be included when writing the attribute):

{
    "operation" => "write-attribute",
    "address" => [
        ("host" => "master"),
        ("server-config" => "server-1"),
        ("jvm" => "default")
    ],
    "name" => "environment-variables",
    "value" => [
        {"foo1" => "bar1"},
        {"foo2" => "bar2"}
    ]
}

Operations write-attribute and undefine-attribute have to be used.