GitHub Workflows security hardening

[sashashura]

This PR adds explicit permissions section to workflows. This is a security best practice because by default workflows run with extended set of permissions (except from on: pull_request from external forks). By specifying any permission explicitly all others are set to none. By using the principle of least privilege the damage a compromised workflow can do (because of an injection or compromised third party tool or action) is restricted.
It is recommended to have most strict permissions on the top level and grant write permissions on job level case by case.

[exceptionfactory]

Thanks for the contribution @sashashura, this looks like a helpful improvement to the workflow configurations.

As noted in the Pull Request Template, all changes need to have an associated Apache NiFi Jira issue for tracking. The Apache NiFi Jira project is available at the following location:

https://issues.apache.org/jira/projects/NIFI

If you can create an issue and update this pull request with the reference issue number, that would be helpful.

[exceptionfactory]

Thanks again for the contribution @sashashura! I am closing this PR in favor of #6469 with associated Jira issue NIFI-10575, but I listed you as a co-author in the commit message.