perf data: Add tracepoint fields when converting to JSON

When converting recorded data into JSON format, perf data omits probe
variables. Add them to the output in the format "field name": "field value"
using tep_print_field:

    $ perf data convert --to-json output.json

    // output.json
    {
        "linux-perf-json-version": 1,
        "headers": { ... },
        "samples": [
            {
            "timestamp": 29182079082999,
            "pid": 309194,
                    [...]
            "__probe_ip": "0x93ee35",
            "query_string_string": "select 2;",
            "nxids": "0"
            }
        ]
    }

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221109103932.65675-1-9erthalion6@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>