fix: JSON arg parsing in CLI

[jannschu]

Hi,

the --input and --data parameters for the CLI are delcared as:

struct Cli {
    // ...
    /// JSON literal to use as data
    #[arg(short, long = "data", group = "data", value_name = "JSON")]
    data_value: Option<serde_json::Value>,

This results in unexpected parsing behavior.

With this declaration clap's value_parser defaults to the From<String> implementation for serde_json::Value to convert the provided argument. The From<String> always results in serde_json::Value::String(...), it does not parse a string containing serialized JSON data, which is not intended I assume.

This, to the best of my knowledge, does prevent users from specifying a JSON object, everything will be treated as a string.

I added a explicit value_parser attribute such that serde_value::from_str is used, which tries to parse a string (unfortunate naming maybe in this context).

# before: parsed as "{}", a JSON string
# with this PR: parsed as {} a JSON object
-i "{}"