feat: Structs mapper utility

[kolea2]

Originally implemented in https://github.com/googleapis/java-logging/blob/d3f897c26b5b55436d2212a0d7452940f15c413f/google-cloud-logging/src/main/java/com/google/cloud/logging/Structs.java, this is a common utility that other projects can benefit from.

Test is from: https://github.com/googleapis/java-logging/blob/d3f897c26b5b55436d2212a0d7452940f15c413f/google-cloud-logging/src/test/java/com/google/cloud/logging/StructsTest.java

The only difference between the java-logging implementation and this is that builder and class methods in Structs.java have been made public.

[blakeli0]

@kolea2 Can you please create an issue for tracking purpose?

Also can I understand more about your use case? Do you need all of the functionality from this class or only a subset of it? Is there any other ways to do it?
In general, it's good to have utils in the common layer, but we also want to be careful as anything we put in common layers will be exposed to all services. So it's much less flexible to make changes in the future and have to be supported (almost)forever.

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