Currently, GtS only supports using the built-in LE client directly for
TLS. However, admins may still want to use GtS directly (so without a
reverse proxy) but with certificates provided through some other
mechanism. They may have some centralised way of provisioning these
things themselves, or simply prefer to use LE but with a different
challenge like DNS-01 which is not supported by autocert.
This adds support for loading a public/private keypair from disk instead
of using LE and reconfigures the server to use a TLS listener if we
succeed in doing so.
Additionally, being able to load TLS keypair from disk opens up the path
to using a custom CA for testing purposes avoinding the need for a
constellation of containers and something like Pebble or Step CA to
provide LE APIs.
* start pulling out + replacing urfave and config
* replace many many instances of config
* move more stuff => viper
* properly remove urfave
* move some flags to root command
* add testrig commands to root
* alias config file keys
* start adding cli parsing tests
* reorder viper init
* remove config path alias
* fmt
* change config file keys to non-nested
* we're more or less in business now
* tidy up the common func
* go fmt
* get tests passing again
* add note about the cliparsing tests
* reorganize
* update docs with changes
* structure cmd dir better
* rename + move some files around
* fix dangling comma
