com.openelements.hiero.spring.implementation.MirrorNodeClientImpl is missing some implementations

[hendrikebbers]

The com.openelements.hiero.spring.implementation.MirrorNodeClientImpl class provides an implementation for spring to call the REST endpoints of a Hiero mirror node instance. The general API can be found at the interface com.openelements.hiero.base.mirrornode.MirrorNodeClient. The spring implementation currently has some methods that only throw a RuntimeException("Not implemented"). Those methods should be implemented and backed by tests.