Breeze should load local tmux configuration in 'breeze start-airflow'

[pateash]

closes #15416

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Description

Currently, when we run

breeze start-airflow

breeze doesn't load local tmux configuration file .tmux.conf and we get default tmux configuration inside the containers.

PR effect
After this change developers will be able to put their .tmux.conf file as /files/airflow-breeze-config/.tmux.conf
and Breeze will load local tmux configuration in to the containers and developers should be able to use their local configurations.

[potiuk]

I guess, I have to update docs as well.

Indeed. Would be great to add it to BREEZE.rst, though we print it as output. I think we do not have the .env file mentioned there - so a small "customizing" environment chapter would be nice.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.