Updating services xml schema file to support additional tag attrs 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT

I noticed in the XML schema file for services, the `<tag />` tag does not support the following attributes:

- event
- method
- priority

I have updated the schema to support these attributes.