[feat] Support ExclusiveWithFencing producer access mode.

[shibd]

Motivation

#95

Modifications

• Support ExclusiveWithFencing producer access mode.

Verifying this change

• Add testExclusiveWithFencingProducer to cover it.

Documentation

• doc-required
  (Your PR needs to update docs and you will update later)

• doc-not-needed
  (Please explain why)

• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)

[Anonymitaet]

Hi @shibd, thanks for introducing this great feature!

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As we discussed just now, for the doc side:

✅ Pulsar feature matrix

I've ticked the 4 boxes of "Access Mode" for C++.

✅ Pulsar client docs

🌈 Collobration way

To add docs for it, feel free to choose your preferred method (either way works for me):

• You provide tech inputs to me. I'll write docs based on them and request your review. Or

• You submit a PR to the pulsar-site repo directly and request my review.

🌈 TO-DO tasks

1. Create a topic named "Enable access mode" in work with producers. It should explain:

• What is it? (we already have this conceptual content, just add the link to this topic)
• How to use it? (e.g., any considerations? Sample code, etc.)

2. This doc is added to 5 doc versions (NEXT, 2.11.x, 2.10.x, 2.9.x, 2.8.x) since this feature is available from Pulsar 2.8.x.

• Original doc file of NEXT is here.
• Original doc files of all versioned docs are here.

We can update one version first and apply the same doc changes to the rest once it gets approved.

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Feel free to reach out to me if you have questions. TIA!