### 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
final PutFollowAction.Request followRequest = putFollow("index1", "index2");
        PutFollowAction.Response response = followerClient().execute(PutFollowAction.INSTANCE, followRequest).get();
        assertTrue(response.isFollowIndexCreated());
        assertTrue(response.isFollowIndexShardsAcked());
        assertTrue(response.isIndexFollowingStarted());
        final PutFollowAction.Request followRequest2 = putFollow("index1", "index2");
        expectThrows(SnapshotRestoreException.class, () -> followerClient().execute(PutFollowAction.INSTANCE, followRequest2).actionGet());
        followerClient().admin().indices().prepareClose("index2").get();
        expectThrows(
            ResourceAlreadyExistsException.class,
            () -> followerClient().execute(PutFollowAction.INSTANCE, followRequest2).actionGet()
        );
    }
    public void testIndexFallBehind() throws Exception {
        runFallBehindTest(() -> {
            
            final ClusterStateResponse followerIndexClusterState = followerClient().admin()
                .cluster()
                .prepareState()
                .clear()
                .setMetadata(true)
                .setIndices("index2")
                .get();
            final String followerUUID = followerIndexClusterState.getState().metadata().index("index2").getIndexUUID();
            final ClusterStateResponse leaderIndexClusterState = leaderClient().admin()
                .cluster()
                .prepareState()
                .clear()
                .setMetadata(true)
                .setIndices("index1")