Look into removing BouncyCastle dependency

[jtattermusch]

This dependency was originally introduced to allow parsing PKCS8 encoded keys that are part of .json credential files (support added in 1.9.3).
Reimplementing PKCS8 parsing or finding a more lightweight dependency to take would allow us to drop the somewhat biggish BouncyCastles dependency.
https://www.nuget.org/packages/BouncyCastle/

[manigandham]

There are better libraries like https://github.com/adamcaudill/libsodium-net or http://securitydriven.net/inferno which can replace BouncyCastle.

[jtattermusch]

Thanks, Chris!