fix(material/core): allow optgroup overrides through core-overrides (angular#29897)

Fixes that we were supporting overrides for `mat-option` through the `core-overrides` mixin, but not for `mat-optgroup`.

Author: crisbeto

src/material/core/_core-theme.scss

@@ -75,13 +75,15 @@
   $app-tokens: tokens-mat-app.get-token-slots();
   $ripple-tokens: tokens-mat-ripple.get-token-slots();
   $option-tokens: tokens-mat-option.get-token-slots();
+  $optgroup-tokens: tokens-mat-optgroup.get-token-slots();
   $full-pseudo-checkbox-tokens: tokens-mat-full-pseudo-checkbox.get-token-slots();
   $minimal-pseudo-checkbox-tokens: tokens-mat-minimal-pseudo-checkbox.get-token-slots();
 
   @return (
     (namespace: tokens-mat-app.$prefix, tokens: $app-tokens, prefix: 'app-'),
     (namespace: tokens-mat-ripple.$prefix, tokens: $ripple-tokens, prefix: 'ripple-'),
     (namespace: tokens-mat-option.$prefix, tokens: $option-tokens, prefix: 'option-'),
+    (namespace: tokens-mat-optgroup.$prefix, tokens: $optgroup-tokens, prefix: 'optgroup-'),
     (
       namespace: tokens-mat-full-pseudo-checkbox.$prefix,
       tokens: $full-pseudo-checkbox-tokens,