feat: Allow audience to be explicitly specified

The default audience for the GitHub OIDC uses sts.amazonaws.com, but there are
situations when it would be desirable to allow different audience names to be
used instead. Allow this to be specified as an argument to the action.

index.js

@@ -19,6 +19,7 @@ async function assumeRole(params) {
   const isDefined = i => !!i;
 
   const {
+    audience,
     sourceAccountId,
     roleToAssume,
     roleExternalId,
@@ -263,6 +264,7 @@ async function run() {
   try {
     // Get inputs
     const accessKeyId = core.getInput('aws-access-key-id', { required: false });
+    const audience = core.getInput('audience', { required: false });
     const secretAccessKey = core.getInput('aws-secret-access-key', { required: false });
     const region = core.getInput('aws-region', { required: true });
     const sessionToken = core.getInput('aws-session-token', { required: false });
@@ -310,7 +312,7 @@ async function run() {
     let sourceAccountId;
     let webIdentityToken;
     if(useGitHubOIDCProvider()) {
-      webIdentityToken = await core.getIDToken('sts.amazonaws.com');
+      webIdentityToken = await core.getIDToken(audience);
       roleDurationSeconds = core.getInput('role-duration-seconds', {required: false}) || DEFAULT_ROLE_DURATION_FOR_OIDC_ROLES;
       // We don't validate the credentials here because we don't have them yet when using OIDC.
     } else {