add aegis support

[kazuho]

This is the h2o-side adjustments for aegis support, based on h2o/picotls#478.

Specifically, this PR makes the following changes:

• adds libaegis to the Ubuntu 22.04 CI image
• link to libaegis if specified (default is on if libaegis is available)
• detect and enable aegis cipher suites if configured (e.g., cipher-suite-tls1.3: [ TLS_AEGIS_128L_SHA256 ])

The test being added merely checks if the server boots correctly with aegis turned on. If we want to go a step further, we might consider the following:

• build picotls-cli (the TLS 1.3 test client that we build inside the h2o repo) with aegis support, and
• use that to actually test HTTP/1.1 over TLS/1.3 using aegis.

For picotls-cli, it would be best to adjust picotls so that aegis support will be on by default when libaegis is found.

[kazuho]

@jedisct1 WDYT?

[jedisct1]

LGTM!

[kazuho]

@jedisct1 Thank you for checking! Merged.