### 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
equalTo("no shard follow tasks for [follower]")
        );
        assertThat(
            expectThrows(
                IllegalArgumentException.class,
                () -> followerClient().execute(PauseFollowAction.INSTANCE, new PauseFollowAction.Request("regular-index")).actionGet()
            ).getMessage(),
            equalTo("index [regular-index] is not a follower index")
        );
        assertThat(
            expectThrows(
                IndexNotFoundException.class,
                () -> followerClient().execute(PauseFollowAction.INSTANCE, new PauseFollowAction.Request("xyz")).actionGet()
            ).getMessage(),
            equalTo("no such index [xyz]")
        );
    }
    public void testUnfollowIndex() throws Exception {
        String leaderIndexSettings = getIndexSettings(1, 0);
        assertAcked(leaderClient().admin().indices().prepareCreate("index1").setSource(leaderIndexSettings, XContentType.JSON).get());
        PutFollowAction.Request followRequest = putFollow("index1", "index2");
        followerClient().execute(PutFollowAction.INSTANCE, followRequest).get();
        leaderClient().prepareIndex("index1").setSource("{}", XContentType.JSON).get();
        assertBusy(() -> { assertThat(followerClient().prepareSearch("index2").get().getHits().getTotalHits().value, equalTo(1L)); });
        
        
        
        pauseFollow("index2");
        followerClient().admin().indices().close(new CloseIndexRequest("index2").masterNodeTimeout(TimeValue.MAX_VALUE)).actionGet();
        assertAcked(followerClient().execute(UnfollowAction.INSTANCE, new UnfollowAction.Request("index2")).actionGet());