fix(MultipartController): use message recipient as token audience

[ronjaquensel]

What this PR changes/adds

The MultipartController now uses the recipient of a response message as the audience for the token request.

Why it does that

Previously, the issuer of the message was used. Thus, the connector used itself as the token audience for responses.

Linked Issue(s)

Closes #2017

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)