AIP84: UI endpoint for config

[vatsrahul1001]

closes: https://github.com/apache/airflow/issues/43166

Description

In the old UI, we passed a lot of variables via global variables in FAB templates. This PR replaces that with ui/config so that UI can use it.

Response json

{
    "navbar_color": "#fff",
    "navbar_text_color": "#51504f",
    "navbar_hover_color": "#eee",
    "navbar_text_hover_color": "#51504f",
    "navbar_logo_text_color": "#51504f",
    "page_size": 100,
    "auto_refresh_interval": 3,
    "default_ui_timezone": "UTC",
    "hide_paused_dags_by_default": false,
    "instance_name": "Airflow",
    "instance_name_has_markup": false,
    "enable_swagger_ui": true,
    "require_confirmation_dag_change": false,
    "default_wrap": false,
    "warn_deployment_exposure": true,
    "audit_view_excluded_events": "",
    "audit_view_included_events": "",
    "is_k8s": false,
    "test_connection": "Disabled",
    "state_color_mapping": {
        "deferred": "mediumpurple",
        "failed": "red",
        "queued": "gray",
        "removed": "lightgrey",
        "restarting": "violet",
        "running": "lime",
        "scheduled": "tan",
        "skipped": "hotpink",
        "success": "green",
        "up_for_reschedule": "turquoise",
        "up_for_retry": "gold",
        "upstream_failed": "orange"
    }
}

Testing

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[vatsrahul1001]

Looking good.

Just a few suggestions/questions.

Are we missing the standalone_dag_processor ?

As per below inline from Brent we have to do this in issue

Other config:
standalone_dag_processor [This should be checked in API, not the UI](https://github.com/apache/airflow/issues/44253)

[pierrejeambrun]

Are we missing the standalone_dag_processor ?

At first glance, it feels like we just need to add an extra field to additional_config with conf.getboolean("scheduler", "standalone_dag_processor") ?

Maybe i'm missing something though.