Fixes #217.

A port of AES-GCM cipher support from Apache MINA-SSHD, based on apache/mina-sshd#132.

Included tests for decoding SSH packets sent from Apache MINA-SSHD and OpenSSH (Version 7.9p1 as used by Debian 10).

Manual tests also done on OpenSSH server 7.9p1 running Debian 10 with its available ciphers, including 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, aes128-gcm@openssh.com and aes256-gcm@openssh.com.

- Fixed variable/statement whitespaces and add back missing braces per coding standard requirement
- Moved Buffer.putLong() and Buffer.getLong() into GcmCipher.CounterGCMParameterSpec since it's the only user
- Moved BaseCipher.authSize into GcmCipher since it is the only cipher that would return a non-zero. BaseCipher will keep return 0 instead
- Made BaseCipher.cipher protected instead of making it publicly accessible
- Combined the three decoding modes in Decoder.decode() into one single method, to reduce code duplication
- Added integration test for the ciphers, along with the newly implemented AES-GCM ciphers