…led by dagcached_enabled and only_from_file

…artial

…Operator

Fix CI by filtering DAG list and removing SubDAGs

Figured out that the issue was because the it tries to remove SubDAG from DB which does not exist as a separate row and hence fails.

- Since it is now in a single document we can use internal schema
references to reuse common chunks

- Don't store dag_id against for serialized tasks

  We never store serialized operators apart from their dags, so we don't
  need to include the dag_id in the structure again. This change means
  we need to re-associate the task and the dag after inflating the dag,
  but does also mean we don't need to pass `visited_dags` around all over
  the place

- Always serialize template_fields as an list, never an array

  Although many operator classes set these as tuples instead of lists,
  that is a distinction that is not important to us here, and makes the
  schema more complex.

Don't store defaults, and remove many (but not all) of the type
annotations. in the JSON). Combined this reduces the size of our large
test DAGs to 40% of what they were

(cherry picked from commit a590ed3253f1f41e46a30df966cf17d5dae989f3)

(cherry picked from commit 073464a17ab1b997b50994df7513b57e99f570c5)

This was a valid type for schedule_interval already, so we should
continue supporting it

(cherry picked from commit ec9d705f1a90790bdcb099196269c77d3cc3d53c)

To save start-up time (and memory) this changes the DabBag to not be
populated by the webserver on start up - and when a specific dag is
asked for it will be loaded on-demand from the SerializedDAG table.

Co-Authored-By: Ash Berlin-Taylor <ash_github@firemirror.com>

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