Add optional SSL key logging?

[Keno]

In order to debug when things go wrong, it would be useful to be able to analyze the traffic in Wireshark. Of course, in general this traffic will be encrypted. However, Wireshark supports reading in the encryption key and decrypting the traffic: https://wiki.wireshark.org/TLS. It would be great if HTTP.jl could optionally write out those keys so we can look at the traffic in wirewhark.

[Keno]

Ah, I see MbedTLS already has secrets_log_file. Let me try that.

[Keno]

For my own future use, this worked fine:

@@ -652,8 +654,8 @@ function global_sslconfig(require_ssl_verification::Bool)::SSLConfig
     global default_sslconfig
     global noverify_sslconfig
     if default_sslconfig === nothing
-        default_sslconfig = SSLConfig(true)
-        noverify_sslconfig = SSLConfig(false)
+        default_sslconfig = SSLConfig(true; log_secrets="/home/keno/secrets.log")
+        noverify_sslconfig = SSLConfig(false; log_secrets="/home/keno/secrets.log")
     end
     return require_ssl_verification ? default_sslconfig : noverify_sslconfig
 end