### 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
leaderClient().prepareIndex("leader").setSource("{}", XContentType.JSON).get();
        }
        assertBusy(
            () -> assertThat(followerClient().prepareSearch("follower").get().getHits().getTotalHits().value, equalTo(firstBatchNumDocs))
        );
        
        {
            GetSettingsRequest getSettingsRequest = new GetSettingsRequest();
            getSettingsRequest.indices("follower");
            GetSettingsResponse getSettingsResponse = followerClient().admin().indices().getSettings(getSettingsRequest).actionGet();
            assertThat(getSettingsResponse.getSetting("follower", "index.number_of_replicas"), equalTo("0"));
        }
        assertThat(getFollowTaskSettingsVersion("follower"), equalTo(1L));
        UpdateSettingsRequest updateSettingsRequest = new UpdateSettingsRequest("leader");
        updateSettingsRequest.settings(Settings.builder().put("index.number_of_replicas", 1));
        assertAcked(leaderClient().admin().indices().updateSettings(updateSettingsRequest).actionGet());
        final int secondBatchNumDocs = randomIntBetween(2, 64);
        for (long i = firstBatchNumDocs; i < firstBatchNumDocs + secondBatchNumDocs; i++) {
            leaderClient().prepareIndex("leader").setSource("{}", XContentType.JSON).get();
        }
        assertBusy(() -> {
            GetSettingsRequest getSettingsRequest = new GetSettingsRequest();
            getSettingsRequest.indices("follower");
            GetSettingsResponse getSettingsResponse = followerClient().admin().indices().getSettings(getSettingsRequest).actionGet();
            assertThat(getSettingsResponse.getSetting("follower", "index.number_of_replicas"), equalTo("0"));
            assertThat(getFollowTaskSettingsVersion("follower"), equalTo(2L));
            try {
                assertThat(
                    followerClient().prepareSearch("follower").get().getHits().getTotalHits().value,
                    equalTo(firstBatchNumDocs + secondBatchNumDocs)