Configurable engine parameters contextEngineID, contextName, and engineBoots

[KristianVirkus]

Implemented configurable engine parameters for engineBoots, contextEngineID, and contextName as additional constructors in class EngineGroup. The known usage of the adapted classes is still possible and still behaves as before.

I'm aware that two of the unit tests fail, but I'm not into ASN.1/SNMP byte internals enough to modify the embedded binary resources in order to match the expected test result.

[KristianVirkus]

Additional changes required to be able to specify contextName and contextEngineId in file "/SharpSnmpLib/Messaging/Discovery.cs" for creating discovery messages?

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