Implement command/REST endpoint for offline tx sign off #1953

* Add sign CLI command to sign transactions generated with the
  --generate-only flag.
* Add /sign REST endpoint for Voyager support.

Redirect password prompt to STDERR to avoid messing up cli
commands output. As a rule of thumb, program's output should
always go to STDOUT, whilst errors&diagnostics go to STDERR
as per POSIX's philosophy and specs.

docs/sdk/clients.md

@@ -159,6 +159,7 @@ gaiacli sign \
   unsignedSendTx.json > signedSendTx.json
 ```
 
+
 ### Staking
 
 #### Set up a Validator