feat: add subscription dispatch rate to namespace resource

[efcasado]

Motivation

At the time of this writing, dispatch_rate can only be used for configuring the dispatch rate for all the topics under a given namespace. This pull-request adds support for configuring the dispatch rate for all the subscriptions under a given namespace, which can come in handy in cases where more granular control is needed to prevent one subscription putting excessive load on a broker.

Modifications

• Added subscription_dispatch_rate to the pulsar_namespace resource
• Updated the documentation to reflect the difference between dispatch_rate and subscription_dispatch_rate

Verifying this change

• Make sure that the change passes the CI checks.

• *This change is already covered by existing tests, such as (please describe tests).

Documentation

Check the box below.

Need to update docs?

• doc-required

  (If you need help on updating docs, create a doc issue)

• no-need-doc

  (Please explain why)

• doc

  (If this PR contains doc changes)

[github-actions]

@efcasado:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@efcasado:Thanks for providing doc info!

[maxsxu]

@efcasado Thanks for your contribution!

[freeznet]

lgtm