Add sq pki path --userid-by-email.

- Add `sq pki path --userid-by-email`, which selects the self-signed
    user ID with the specified email address.  This is different from
    `--email`, which simply uses the argument's value as a user ID.

  - Fixes #212.

src/cli/pki/path.rs

@@ -48,7 +48,7 @@ the specified user ID.
 
     #[command(flatten)]
     pub userids: UserIDDesignators<
-        userid_designator::PlainAddArgs,
+        userid_designator::PlainAddAndByArgs,
         userid_designator::OneValueNoLinting,
         userid_designator::AnyDocumentation>,
 

src/cli/types/userid_designator.rs

@@ -79,6 +79,9 @@ pub type ExactByAndAddArgs
 pub type AllExactByAndAddArgs
     = <AllUserIDsArg as std::ops::BitOr<ExactByAndAddArgs>>::Output;
 
+pub type PlainAddAndByArgs
+    = <ByArgs as std::ops::BitOr<PlainAddArgs>>::Output;
+
 /// Argument parser options.
 
 /// Normally it is possible to designate multiple certificates.  This
@@ -1198,6 +1201,7 @@ mod test {
         check!(ExactAndAddArgs,         Exact, false,  true);
         check!(ExactByAndAddArgs,       Exact,  true,  true);
         check!(AllExactByAndAddArgs,    Exact,  true,  true);
+        check!(PlainAddAndByArgs,         Add,  true, false);
     }
 
     #[test]