Allow omission of initial_node_count if node_pools is specified

[leahecole]

According to the API reference, the initial_node_count should not be used when you are configuring node_pools because you specify the initial_node_count within the node_pools object.

This PR adds logic allowing either initial_node_count OR node_pools and updates test cases. I also refactored the test to make it more human readable because boolean logic makes me dizzy sometimes 😄

[leahecole]

Apologies - I'll fix the static checks later today. I also have a second check I think i need to add to make sure you don't have both an initial node count AND nodepools

[leahecole]

Okay! This is ready. I got myself really tangled in some boolean logic but have since recovered 😅

[mik-laj]

LGTM. However, I think that we can safely delete this validation and rely only on the server-side validation.

PS. Project ID should be optionally, because we can obtain a project ID from credential.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.