feat(publish) Publish Secrets to Azure Key Vault

[dmunch]

Hi,

first, thanks a lot for sops - a great tool which accommodates many of our needs in regards to secret management!

For our specific use-case, we needed to publish secrets to Azure Key Vault Secrets - While the general publish functionality of sops is useful, it currently lacks support to publish secrets to Azure Key Vault Secrets. This PR is an attempt to get this added.

Besides this, this PR also adds support for the input-type command line flag to publish command .

I'm a first-time contributor, and not too experienced Golang, so I hope I did things the right way. Happy to make any changes if required.

Regarding testing, there weren't any existing tests for the other publishers, so I couldn't just easily follow existing examples.

[dmunch]

Thanks for the review, and sorry for the late follow-up. I've added the requested changes to the README.rst file, and merged current main.