Make BasicPublishBatch work with ReadOnlyMemory<byte>

[Samuron]

Hi guys!

Thank you for revamping the API of the client to use new primitives. I've found that BasicPublishBatch still uses byte[] for payload and found it inconsistent.

This is breaking change on the binary level AFAIU, but it should be compatible on recompile due to implicit conversions.

Not sure how it should be handled, feel free to close the PR.

[pivotal-issuemaster]

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[michaelklishin]

I remember @bording or @stebet mentioned that there is an implicit conversion between those types. If that's the case, do we need this change at all? If not, this kind of change can only go into 7.0 as it changes a method signature?

Also highly relevant: #788.

[michaelklishin]

Sorry, I now see the difference here from #788: this updates the batch publishing API.

[lukebakken]

Yes, if accepted, this must be a 7.0 feature. It would have been nice to get this into 6.0 🤷

[danielmarbach]

An alternative non-breaking extension to support BasicPublishBatch #865. This could be added in 6.2

[danielmarbach]

The alternative has been merged. Closing