InRelease file: support Signed-By field

https://wiki.debian.org/DebianRepository/Format#Signed-By says:
> **Signed-By**
> An optional field containing a comma separated list of
> OpenPGP key fingerprints to be used for validating
> the next Release file. The fingerprints must consist
> only of hex digits and may not contain spaces.
> The fingerprint specifies either the key the Release file
> must be signed with or the key the signature key must be
> a subkey of. The later match can be disabled by appending
> an exclamation mark to the fingerprint.
>
> If the field is present, a client should only accept future updates
> to the repository that are signed with keys listed in the field.
> The field should be ignored if the Valid-Until field is not present
> or if it is expired.

For both the CLI tools and JSON, the field is taken as a string verbatim.

When specified, we must also provide `Valid-Until` field,
and i'm not sure there is an 'infinity' value for it,
so 100 years will have to do?

Fixes https://github.com/aptly-dev/aptly/issues/1497

AUTHORS

@@ -77,3 +77,4 @@ List of contributors, in chronological order:
 * Yaksh Bariya (https://github.com/thunder-coding)
 * Juan Calderon-Perez (https://github.com/gaby)
 * Ato Araki (https://github.com/atotto)
+* Roman Lebedev (https://github.com/LebedevRI)