[PATCH 0 of 2] certificate compression

[Sergey Kandaurov]

Notably, long certificate chains are compressed better,
with zlib demonstrating a slightly worse ratio.

no      zlib    brotli  zstd
1       .973    .964    .954
2       .907    .881    .877
3       .877    .853    .849
4       .856    .837    .836
5       .842    .827    .827
6       .835    .821    .822

Further, using ECDSA certificates (which itself produces Certificate
TLS messages of a smaller size compared to RSA, apparently due to
"using keys with small public key representations" (c) RFC 9001)
allows to achieve better compression results.

Applied to QUIC handshake, this may conserve an additional round trip
when using long certificate chains with a not yet validated address.
Testing on self-signed certificates demonstrates an additional round
trip on a 5th RSA and 11th ECDSA certificate, real results may vary.

=== rsa ===
  server datagrams sent   w/ compression   cert msg ratio
1  1252 177               1252 167              .98
2  1252 865               1252 747              .91
3  1252 1252 369          1252 1252 123         .88
4  1252 1252 1057         1252 1252 672         .86
5  1252 1252 1252 - 561   1252 1252 1210        .84
6  1252 1252 1252 - 1248  1252 1252 1252 - 578  .84

=== ecdsa ===

1  1200                   1200                  .90
2  1200                   1200                  .65
3  1252 178               1200                  .56
4  1252 470               1200                  .51
5  1252 760               1200                  .48
6  1252 1053              1252 111              .47
7  1252 1252 158          1252 218              .45
8  1252 1252 450          1252 322              .44
9  1252 1252 740          1252 426              .43
A  1252 1252 1033         1252 529              .42
B  1252 1252 1252 - 139   1252 631              .42
C  1252 1252 1252 - 431   1252 737              .41

Feedback is welcome.