Add relative path support for Protobuf import path

[stefan-hudelmaier]

Currently, the import path for Protobuf definition files is an absolute one. It would be great if it were possible to specify relative paths here. They would have to be relative to configuration file. Example:

contexts:
    somecontext:
        brokers:
            - broker.example.com:9093
        sasl:
            enabled: true
            mechanism: oauth
            tokenprovider:
                plugin: azure
        tls:
            enabled: true
        protobuf:
            importPaths:
             - ./proto-definitions
            protoFiles:
             - message.proto