BoringSSL: Switch SecureSocket implementation from NSS to BoringSSL

[whesse]

We are switching the Dart standalone executable "dart" and the dart:io library to use the BoringSSL library (https://boringssl.googlesource.com/boringssl/)
instead of the Mozilla NSS library.

This will simplify the API for importing certificates and keys, simplify the build process and decrease the executable size, and switch to a simpler, smaller SSL library that is maintained by Google with outdated code removed and new TLS features added quickly.

• Merge in BoringSSL branch
• Support onBadCert arg: BoringSSL - Implement bad certificate callback for client SecureSocket connection #23561
• Add change log entry
• Update documentation
• Blog post
• Report errors from multithreaded encrypting: Return BoringSSL error messages from multithreaded encryption processing #24185

This master bug (issue) will be used to track requests, and be marked as blocked on all the bugs tracking individual feature implementations. @mit-mit: edited this to fit other meta issues.

[whesse]

Marked this as being blocked by #23561.

[Scorpiion]

Looking forward to this, a nice change!

[mit-mit]

@whesse, I updated this to follow the meta bug style. Can you complete the list I started for things that need to happen in 1.13? Thanks!