Update tinkerhost.net.tinkermail.json (#590)

Adds additional MX record

# Description

Adds an additional MX record to be added at base domain level

## Type of change

Please mark options that are relevant.

- [ ] New template
- [ ] Bug fix (non-breaking change which fixes an issue in the template)
- [x] New feature (non-breaking change which adds functionality to the
template)
- [ ] Breaking change (fix or feature that would cause existing template
behavior to be not backward compatible)

# How Has This Been Tested?

Please mark the following checks done
- [ ] Schema validated using JSON Schema
[template.schema](./template.schema)
- [ ] Template functionality checked using [Online
Editor](https://domainconnect.paulonet.eu/dc/free/templateedit)
- [x] Template is checked using [template
linter](https://github.com/Domain-Connect/dc-template-linter)
- [x] Template file name follows the pattern
`<providerId>.<serviceId>.json`

Online Editor shows the error `Error! A problem has been occurred while
submitting your data: Invalid data for TXT host:
tinkermail_verify.testdomain.com (from tinkermail_verify)`, ignoring as
the TXT record has not been changed and has been previously approved and
accepted.

# Example variable values


```
cname1: odezzviqnsius6mqegpnn6ni3pofxbtj
cname1: odezzviqnsius6mqegpnn6ni3pofxbtj
cname1: odezzviqnsius6mqegpnn6ni3pofxbtj
txtverif: mailN2JhNjJkODA1YjZiODE1NjBlMjMzOWFlN2Q0YTc3MzU=
```

tinkerhost.net.tinkermail.json

@@ -3,7 +3,7 @@
   "providerName": "TinkerHost",
   "serviceId": "tinkermail",
   "serviceName": "TinkerHost TinkerMail",
-  "version": 2,
+  "version": 3,
   "syncPubKeyDomain": "domainconnect.tinkerhost.net",
   "syncRedirectDomain": "app.tinkerhost.net",
   "logoUrl": "https://app.tinkerhost.net/assets/image/th.svg",
@@ -46,6 +46,13 @@
       "priority": 10,
       "ttl": 3600
     },
+    {
+      "type": "MX",
+      "host": "@",
+      "pointsTo": "mailmx.tinkerhost.net",
+      "priority": 10,
+      "ttl": 3600
+    },
     {
       "type": "TXT",
       "host": "tinkermail_verify",