add support for label_join and label_replace function

[yeya24]

Fixes #29, this PR adds fuzz support for label_join and label_replace.

The destination label is hardcoded to be __promqlsmith_dst_label__ and it is expected for users to not use this label in their series.

The source labels used for join and replace are picked in random. Label join shouldn't have more than 2 labels for joining for simplicity.

label_join(<randomly generated vector selector>, "__promqlsmith_dst_label__", ",", "<source_label1>", "<source_label2>")

Label replace is only copying same label to target label for simplicity.

label_replace(<randomly generated vector selector>, "__promqlsmith_dst_label__", "$1", "<source_label>", "(.*)")