DPI bypass tools like ByeDPI fragment a single TLS record into multiple
records to evade censorship. This broke ReadClientHello because it
assumed the entire ClientHello arrives in one TLS record.

Add reassembleTLSHandshake that reads continuation records and
reconstructs a single TLS record before parsing and HMAC verification.
Per RFC 5246 Section 6.2.1, handshake messages may be fragmented
across multiple records — this is valid TLS behavior.

This helps with a situation when some applications do not allow mounting
individual files, but whole directories. In that case users could mount
`/config` directory with a single file, `config.toml`: `-v
/path/to/dir:/config`. Also, there is a backward compatibility to using
a single `/config.toml`