Add multicast option to request node

[JKRhb]

This PR adds a multicast option to the request node. Before, it was not possible to perform actual multicast requests as the underlying node-coap package requires this option to be set in requests to do so. The PR is still WIP as new unit tests and the possibility to set the multicast timeout are still missing.

[coveralls]

Pull Request Test Coverage Report for Build 603686128

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 90.96%

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Totals
Change from base Build 603403890:	0.2%
Covered Lines:	117
Relevant Lines:	126

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💛 - Coveralls