sign: add new builtin for signing

sign: add new builtin for signing

This builtin allows to sign and verify commit with new signature
mechanism. At the moment it is possible to use 'dummy' and 'ed25519'
signing modules.

'dummy' module use any ASCII string from command line as a key for
commit's signing or verification.

Support of ed25519 signature is implemented with `libsoium` library.
Secret and public key should be provided in hex presentation via
command line.

Based on 'gpg-sign' source.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>