Support utf8mb4_unicode_520_ci

## Enhancement

TiDB could support `utf8mb4_unicode_520_ci`. This collation is supported since MySQL 5.7.0.

utf8_unicode_ci is based on UCA 4.0.0 weight keys (http://www.unicode.org/Public/UCA/4.0.0/allkeys-4.0.0.txt).

utf8_unicode_520_ci is based on UCA 5.2.0 weight keys (http://www.unicode.org/Public/UCA/5.2.0/allkeys.txt).

Ref: https://stackoverflow.com/questions/37307146/difference-between-utf8mb4-unicode-ci-and-utf8mb4-unicode-520-ci-collations-in-m

Some MySQL applications like Wordpress uses this collation by default, as a substitute to `utf8mb4_unicode_ci`:

```sql
CREATE TABLE `posts` (
`ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_author` bigint(20) unsigned NOT NULL DEFAULT 0,
`post_date` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`post_date_gmt` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`post_content` longtext COLLATE utf8mb4_unicode_520_ci NOT NULL,
`post_title` text COLLATE utf8mb4_unicode_520_ci NOT NULL,
`post_excerpt` text COLLATE utf8mb4_unicode_520_ci NOT NULL,
`post_status` varchar(20) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT 'publish',
`comment_status` varchar(20) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT 'open',
`ping_status` varchar(20) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT 'open',
`post_password` varchar(255) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT '',
`post_name` varchar(200) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT '',
`to_ping` text COLLATE utf8mb4_unicode_520_ci NOT NULL,
`pinged` text COLLATE utf8mb4_unicode_520_ci NOT NULL,
`post_modified` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`post_modified_gmt` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`post_content_filtered` longtext COLLATE utf8mb4_unicode_520_ci NOT NULL,
`post_parent` bigint(20) unsigned NOT NULL DEFAULT 0,
`guid` varchar(255) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT '',
`menu_order` int(11) NOT NULL DEFAULT 0,
`post_type` varchar(20) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT 'post',
`post_mime_type` varchar(100) COLLATE utf8mb4_unicode_520_ci NOT NULL DEFAULT '',
`comment_count` bigint(20) NOT NULL DEFAULT 0,
PRIMARY KEY (`ID`),
KEY `post_name` (`post_name`(191)),
KEY `type_status_date` (`post_type`,`post_status`,`post_date`,`ID`),
KEY `post_parent` (`post_parent`),
KEY `post_author` (`post_author`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci AUTO_INCREMENT=124960
```

Currently TiDB reports errors like:

ERROR 1273 (HY000): Unsupported collation when new collation is enabled: 'utf8mb4_unicode_520_ci'

### Development tasks

- [ ] TiKV supports utf8mb4_unicode_520_ci
- [ ] TiFlash supports utf8mb4_unicode_520_ci
- [ ] TiDB server supports utf8mb4_unicode_520_ci