Previously zstd-codec included some stub code from emscripten that
automatically hooked uncaught exceptions & rejections, which could cause
problems. This only affected usage when zstd was used, since it's loaded
lazily, but would break environments (such as test suites) where
uncaught errors aren't expected to crash the process.

In addition, this adds support for content-encoding: base64, a
non-standard but common encoding, and adds an explicit dev dependency
(previously implicit) on zstd-codec for the Mockttp test suite.

This rule adds additional CAs who should be trusted in addition to the
existing built-in certificate authorities, when this rule connects to
upstream hosts.

Previously, passing { cert: string } or { certPath: string } worked fine
everywhere, but { cert: Buffer } only worked in local clients, due to
serialization issues. We now serialize buffers correctly so that this
works as intended.