feat: Send welcome mail via SOAP call

[sKarki999]

Extends the assignCourseMember() SOAP method to send a welcome email to the member assigned to a course via SOAP.
The "Welcome Mail" setting in the course is respected meaning if disabled, no email is sent.

Expected Behaviour:

1. Welcome Mail 'ON' in course setting
   via UI: Mail Sent
   via SOAP: Mail Sent

2. Welcome Mail 'OFF' in course setting
   via UI: No Mail
   via SOAP: No Mail

To test:

curl -X POST http://<ilias-URL>/soap/server.php \
  -H "Content-Type: text/xml;charset=UTF-8" \
  -d '<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:ilUserAdministration">
         <soapenv:Header/>
         <soapenv:Body>
            <urn:assignCourseMember>
               <sid>REPLACE_SID::REPLACE_CLIENT</sid>
               <course_id>REPLACE_COURSE_REF_ID</course_id>
               <user_id>REPLACE_USER_ID</user_id>
               <type>REPLACE_WITH_TYPE</type>
            </urn:assignCourseMember>
         </soapenv:Body>
      </soapenv:Envelope>'

[Feature Request: Already presented in JF and accepted]

[GitHamo]

LGTM! Thank you very much 👍