Separate the LTI 1.3 access token audience and URL.

This adds a new LTI 1.3 authentication parameter named
`$LTI{v1p3}{AccessTokenAUD}`.  This is used for the audience (`aud`) in
the signed JWT that is sent when requesting an access token from the
LMS.  This access token is used for grade passback.

Previously the `$LTI{v1p3}{AccessTokenURL}` was used for both the
audience and the actual URL that the access token request containing the
signed JWT is sent to.  I suspect that the audience and the URL may not
be the same for all LMS's.  They are the same for Moodle.  These also
needed to be the same for testing on my local Canvas instance.  However,
@Alex-Jordan showed me some information from D2L that indicates these
are different there.  I suspect these may need to be different for
Canvas in production as well.

conf/authen_LTI_1_3.conf.dist

@@ -100,6 +100,7 @@ $LTI{v1p3}{ClientID}        = '';
 $LTI{v1p3}{DeploymentID}    = '';
 $LTI{v1p3}{PublicKeysetURL} = '';
 $LTI{v1p3}{AccessTokenURL}  = '';
+$LTI{v1p3}{AccessTokenAUD}  = '';
 $LTI{v1p3}{AuthReqURL}      = '';
 
 # In the process of LTI 1.3 authentication a request is sent to the LMS in response to its

lib/WeBWorK/Authen/LTIAdvantage/SubmitGrade.pm

@@ -137,7 +137,7 @@ async sub get_access_token ($self) {
 	my $jwt = eval {
 		encode_jwt(
 			payload => {
-				aud => $ce->{LTI}{v1p3}{AccessTokenURL},
+				aud => $ce->{LTI}{v1p3}{AccessTokenAUD},
 				iss => $c->url_for('root')->to_abs->to_string,
 				sub => $ce->{LTI}{v1p3}{ClientID},
 				jti => $private_key->{kid}