feature: rate limit

[SkohTV]

Fix #215

I've started getting into adding this features.
So far, I've managed to add the ratelimit [int]/[time_unit] keyword to the cli and put it's value into a bf_map.
However I'm not sure about implementing a "real" rate limiting as it seems to require EMIT (and that look scary).
Added (empty for now) elfstub to handle rate limiting

The idea of the implementation so far is pretty naive (only allow the first X requests to pass through), mostly because it was easier to implement and I hadn't strong feelings on which direction to go.

I'll keep this as a draft for now since it's still in its early stages.

[qdeslandes]

I went for an early review, mostly for the parsing part. On the BPF side, the map will contain the runtime values for a given rule's rate limit (current burst/allowance, limit, last update time...).

[qdeslandes]

We should expect any float value (even negative, we'll error out later on). We also need a time window , e.g. 10/second.

[SkohTV]

Do we really wanna handle float or do we also error them out later on ?
It feels weird writing 12.1/second (even though it is technically correct), and we could have issues with float precision in some cases.

[qdeslandes]

We don't want to handle floats, but it's easier to error out when parsing the matcher payload (e.g. with a clear error message) than in the lexer.