refactor: use argp library for parsing command line options and arg…

…uments

This commit changes the command line parsing code of `valkey-benchmark`
to use the `argp` library (https://www.gnu.org/software/libc/manual/html_node/Argp.html),
which is included in libc.

This makes the options and arguments parsing more structured and easy to
extend.

Signed-off-by: Ricardo Dias <[email protected]>