Support gzip for session recording

[kontsevoy]

Problem

Currently Teleport stores session streams as-is, i.e. uncompressed. This creates unreasonable storage requirements for large organizations with many servers and users.

Proposal

Teleport should support two types of session files. Plain text (current implementation) and gzipped streams. The recorder must switch to always-gzipped mode, while the player should be able to read both plain text (for compatibility purposes). Early tests have shown that sessions with typical ansible/bash scripting over SSH get 20 times (!) smaller when gzipped.

Bonus Points

Also, when a file is closed and the session is over, can we also cryptographically sign it with Teleport CA's key and drop a signature file with the same name next to it? If we do this, tsh play should display a warning if the signature does not match.

[klizhentas]

I would drop the signature/verify part - this is hard to get right with our RSA keys + there is key rotation to think about, etc, I think gzip is spot-on, but signatures - out of scope