* uplink: Add a new flag to set the filepath of the file which is used for
saving the encryption key and rename the one that hold the encryption key and
establish that it has priority over the key stored in the file to make the
configuration usable without having a huge refactoring in test-sim.
* cmd/uplink: Adapt the setup subcommand for storing the user input key to a file
and adapt the rest of the subcommands for reading the key from the key-file when
the key isn't explicitly set with a command line flag.
* cmd/gateway: Adapt it to read the encryption key from the key-file or use the
one passed by a command line flag.
* pkg/process: Export the default configuration filename so other packages which
use the same value can reference to it rather than having it hardcoded.
* Adapt several integrations (scripts, etc.) to consider the changes applied in uplink and cmd packages.
this change removes the cryptopasta dependency.
a couple possible sources of problem with this change:
* the encoding used for ECDSA signatures on SignedMessage has changed.
the encoding employed by cryptopasta was workable, but not the same
as the encoding used for such signatures in the rest of the world
(most particularly, on ECDSA signatures in X.509 certificates). I
think we'll be best served by using one ECDSA signature encoding from
here on, but if we need to use the old encoding for backwards
compatibility with existing nodes, that can be arranged.
* since there's already a breaking change in SignedMessage, I changed
it to send and receive public keys in raw PKIX format, instead of
PEM. PEM just adds unhelpful overhead for this case.
