Add dynamic_startup_nodes parameter to async RedisCluster

[Kakadus]

Pull Request check-list

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

Description of change

The async version of the RedisCluster lacked the dynamic_startup_nodes parameter. This PR adds the parameter to be activated when dynamic DNS endpoints for startup nodes are in use.

Enabling dynamic_startup_nodes fixes #2472, which describes the problem in detail.

#3111 also addresses said issue, but their logic is different to the sync version, while this approach moves the behaviour of the async version closer to the sync version.