Feature/use client hostname

[ostrya]

When using the standalone server in a dockerized environment, using 'localhost' as the mock's hostname leads to routing problems. Thus, from now on the hostname as used in the issuer field is extracted from the 'Host' request header.

As mentioned in #36 the merge of mock-junit and mock-junit5 forces library users to pull both JUnit versions into their project as well. Thus, this change also reverts the deprecation of mock-junit5.

[sonarcloud[bot]]

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