### Description

It is impossible to create a follower index with a `$` character in its name (please note, it is possible to create a regular index with `$` in its name).

This could be reproduced by rest request:
```
PUT /my-index/_ccr/follow
{
    "remote_cluster": "my-remote-cluster",
    "leader_index": "my-index$follower"
}
```

or `CcrIntegTestCase`:

```
    public void testFollowIndex() throws Exception {
        assertAcked(leaderClient().admin().indices().prepareCreate("my$index"));
        setupRemoteCluster();
        final PutFollowAction.Request followRequest = putFollow("my$index", "my$index");
        followerClient().execute(PutFollowAction.INSTANCE, followRequest).get();
    }
```

This is happening as during initial follow phase we restore the leader cluster from the remote using a snapshot restore.
Corresponding request is created with index name added to `RestoreSnapshotRequest#renamePattern` (regexp) without escaping `$`.

x-pack/plugin/ccr/src/internalClusterTest/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java
try {
                assertThat(
                    followerClient().prepareSearch("follower").get().getHits().getTotalHits().value,
                    equalTo(firstBatchNumDocs + secondBatchNumDocs)
                );
            } catch (Exception e) {
                throw new AssertionError("error while searching", e);
            }
        }, 30, TimeUnit.SECONDS);
        assertThat(hasFollowIndexBeenClosedChecker.getAsBoolean(), is(true));
    }
    public void testDoNotReplicatePrivateSettings() throws Exception {
        assertAcked(leaderClient().admin().indices().prepareCreate("leader").setSource(getIndexSettings(1, 0), XContentType.JSON));
        ensureLeaderGreen("leader");
        final PutFollowAction.Request followRequest = putFollow("leader", "follower");
        followerClient().execute(PutFollowAction.INSTANCE, followRequest).get();
        ClusterService clusterService = getLeaderCluster().getInstance(ClusterService.class, getLeaderCluster().getMasterName());
        clusterService.submitUnbatchedStateUpdateTask("test", new ClusterStateUpdateTask() {
            @Override
            public ClusterState execute(ClusterState currentState) {
                final IndexMetadata indexMetadata = currentState.metadata().index("leader");
                Settings.Builder settings = Settings.builder().put(indexMetadata.getSettings()).put("index.max_ngram_diff", 2);
                if (randomBoolean()) {
                    settings.put(PrivateSettingPlugin.INDEX_INTERNAL_SETTING.getKey(), "private-value");
                }
                if (randomBoolean()) {
                    settings.put(PrivateSettingPlugin.INDEX_PRIVATE_SETTING.getKey(), "interval-value");
                }
                final Metadata.Builder metadata = Metadata.builder(currentState.metadata())
                    .put(