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

man/aptly.1

@@ -1565,6 +1565,10 @@ $ aptly publish repo testing
 Options:
 .
 .TP
+\-\fBsigned\-by\fR
+set value for Signed-By field
+.
+.TP
 \-\fBacquire\-by\-hash\fR
 provide index files by hash
 .
@@ -1706,6 +1710,10 @@ $ aptly publish snapshot wheezy\-main
 Options:
 .
 .TP
+\-\fBsigned\-by\fR
+set value for Signed-By field
+.
+.TP
 \-\fBacquire\-by\-hash\fR
 provide index files by hash
 .
@@ -2065,6 +2073,10 @@ This command would switch published repository (with one component) named ppa/wh
 Options:
 .
 .TP
+\-\fBsigned\-by\fR
+set value for Signed-By field
+.
+.TP
 \-\fBbatch\fR
 run GPG with detached tty
 .
@@ -2171,6 +2183,10 @@ $ aptly publish update wheezy ppa
 Options:
 .
 .TP
+\-\fBsigned\-by\fR
+set value for Signed-By field
+.
+.TP
 \-\fBbatch\fR
 run GPG with detached tty
 .