caddytls: ECH key rotation

[mholt]

This change implements ECH key rotation, made possible in Go 1.25 thanks to the GetEncryptedClientHelloKeys callback in tls.Config.

Currently, keys are rotated every 30 days, and are supported for 90 days (so, 60 days after rotation), to give clients time to update without needing to retry.

It's not clear what are actually good intervals for these things, as this is new technology and there isn't much diversity of deployments. But generally, smaller anonymity sets (fewer sites) should rotate a less frequently than big anonymity sets, and I suspect most Caddy instances are smaller.

We can always change the rotation intervals based on feedback/experience.

[mholt]

Let's try something stupid:

@copilot Are there any (other) resource leaks or any security vulnerabilities in this code?

[Copilot]

@mholt I've opened a new pull request, #7363, to work on those changes. Once the pull request is ready, I'll request review from you.