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.

Author: alblue

Diff for: README.md

@@ -98,6 +98,7 @@ The following table describes which identity is used based on which values are s
     - name: Configure AWS Credentials
       uses: aws-actions/configure-aws-credentials@v1
       with:
+        audience: sts.amazonaws.com
         aws-region: us-east-2
         role-to-assume: arn:aws:iam::123456789100:role/my-github-actions-role
         role-session-name: MySessionName
@@ -108,6 +109,7 @@ In this example, the Action will load the OIDC token from the GitHub-provided en
     - name: Configure AWS Credentials
       uses: aws-actions/configure-aws-credentials@v1
       with:
+        audience: sts.amazonaws.com
         aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
         aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
         aws-region: us-east-2

Diff for: action.yml

@@ -4,6 +4,10 @@ branding:
   icon: 'cloud'
   color: 'orange'
 inputs:
+  audience:
+    default: 'sts.amazonaws.com'
+    description: 'The audience to use for the OIDC provider'
+    required: false
   aws-access-key-id:
     description: >-
       AWS Access Key ID. This input is required if running in the GitHub hosted environment.

Diff for: 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 {