Define means for strong extensibility of TPs

[LPardue]

https://quicwg.org/qlog/draft-ietf-quic-qlog-quic-events.html#name-parameters_set defines some of the core QUIC TPs and then says

Additionally, this event can contain any number of unspecified fields. This is to reflect setting of for example unknown (greased) transport parameters or employed (proprietary) extensions.

That's fine but is it weak and doesn't help interoperability much. We know this is going to be an area of innovation - new extensions and grease values. The spec would be improved by having a well-defined "extension bucket" field that TPs can go in, an array of integers tuples would be fine, for example.

This would help something like the qlog crate accomodate different users https://github.com/cloudflare/quiche/blob/master/tools/qlog/src/lib.rs#L1712 and parse input from different loggers.

[rmarx]

So, this turned out to be slightly easier than expected, see #400.