Add NRT for the whole client assembly

[bollhals]

Proposed Changes

Closes #1596

Enables Nullable Reference Types in the client assembly

I realize a bit late that you intended to do this in 8.0 ... unfortunately I already was 80% through when I did see it 💯

You'll have to decide whether it's worth including it or push it after the snap for 7.0

Types of Changes

• [ ] Bug fix (non-breaking change which fixes issue #NNNN)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• [ ] Documentation improvements (corrections, new content, etc)
• [ ] Cosmetic change (whitespace, formatting, etc)

Checklist

• [x] I have read the CONTRIBUTING.md document
• [x] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [ ] All tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in related repositories

Further Comments

How does it work with the PublicAPI thing, does this need adjustment as well?