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.

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.