miniooni: allow arbitrary OONI Run v2 options

[bassosimone]

This diff is about improving upon the ./internal/oonirun package such that we can provide any JSON document as the options for an experiment, rather than limiting ourselves to scalars, as it's currently the case. This change would effectively remove technical debt and enable experiments to specify more complex sets of options through OONI Run v2. In turn, this would also be beneficial for the richer input strategy. Please, note that, after this change, there's no need anymore to use ooni:"..." to mark options that we would like to set. Setting through JSON will always be possible. Instead, now the ooni:"..." tag is just meant to show which options one could set using the -O name=value flag from the command line.