Add support for DTLS fragmentation

[mickel8]

Traditional BIO_s_mem ignores DTLS fragmentation. OpenSSL just appends subsequent datagrams to the BIO_s_mem's buffer, which results in losing packet boundaries. This was greatly pointed out by @spscream in #31 and described by Lorenzo in OpenSSL mailing list: https://mailing.openssl.users.narkive.com/L431ya4W/openssl-users-dtls-fragmentation-and-mem-bio (many thanks guys!)

Ignoring DTLS fragmentation may result in DTLS datagrams being dropped when MTU size is not large enough. The ServerHello message, in our setup, is ~1400 bytes. When using Orange as ISP, the MTU size seems to be ~1300 bytes so the ServerHello message is dropped.

Another scenario where our implementation fails is when we use 4096 bits certificate.

To mitigate this issue, we can:

• use BIO_s_dgram_mem, which respects packet boundaries. This however, seems to be introduced in OpenSSL 3 so the solution might not be supported on older platforms.
• implement custom filter as described in https://mailing.openssl.users.narkive.com/L431ya4W/openssl-users-dtls-fragmentation-and-mem-bio

OpenSSL 1.1 EOL was in September 2023. Ubuntu was providing security fixes in Ubuntu 20.04 until 04.2025 (not including pro subscribers, who are receiving security updates for 10 years). Newer versions, like 22.04, migrated to OpenSSL 3. Hence, we chose the first option (using BIO_s_dgram_mem). If we see anyone still using OpenSSL 1.1, we will consider providing backward compatible solution. Looks like ubuntu-latest with OTP 26 uses OpenSSL 1.1 (that's the config from our CI) so we go for custom filter.

The MTU is hardcoded to 1200.

Resources:

• https://github.com/openssl/openssl/blob/871182d29dc24bbb51ff7a4d952ff9d9868f14b5/crypto/evp/bio_md.c#L4
• https://github.com/meetecho/janus-gateway/blob/505d97d7fee6bc6370f28bf5e51d491cc5716b27/src/dtls-bio.c#L20
• https://github.com/openssl/openssl/blob/29eb7e0689b9c4231d9603a7d19475bed216d779/crypto/bio/bss_dgram_pair.c#L238
• https://chromium.googlesource.com/chromium/deps/openssl/+/f0c775a4f902aca6abb5d4b1d5ba3260adf73541/openssl/include/openssl/bio.h

Fixes #31