It implements the method proposed in AIP-24 to serialize DAG. It will be used in DAG persistency in DB to solve webserver scalability issue.

(cherry picked from commit 2bd1a51ec75f680a6e6e2101bd948a78421a644a)

…scalability (apache#5743)

(cherry picked from commit 7ce34b2a959fc1f8322836f38f474a831e4901a1)

(cherry picked from commit c068c67c48d294a589b58be0d0ad8b657c361a77)
(cherry picked from commit 04fbf2beac57dcf26b118ebbe5a2bf175ce08af8)

…matching date in DAG

(cherry picked from commit be412522cb95a19a51b2f208ae8ebea76e8b667a)

(cherry picked from commit d030b10bec9cd0e468f36e97e131d497d5a43fc6)

- Code Cleanup for JSON columns
- Test code to allow old mysql & sqlite versions

(cherry picked from commit 1db8044f9d29edf25f2b8ad4cd21c496c243534a)

(cherry picked from commit d14497ff28d123d45d626019cabcbd977c5de79d)

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

(cherry picked from commit ec9d705f1a90790bdcb099196269c77d3cc3d53c)
(cherry picked from commit 9805b4a183b87976dc33ae80c7e6a209849ba5d7)
(cherry picked from commit f00d9237cd9224571e43bda67ad4dddfb009c402)

(cherry picked from commit 92d442d33dd8c81ea73026405d3978d133140807)
(cherry picked from commit 7d371d329613c48deef0d8a812c817f2013db8f9)

(cherry picked from commit 549c1f9cd9ab0bfeac4f75fa713cbaae842a6e82)

(cherry picked from commit d0ce27e3f3b6046016800855ad2e57fa67d8b57f)
(cherry picked from commit 50a60b6a026e6d6249f069944be86560d87a67ca)

(cherry picked from commit 5a84ca517cef0dacff23f57b360a554b461b5034)

(cherry picked from commit 712ff47cbada7373eaa4fa92bb9220a453c445ae)
(cherry picked from commit 52a0e9e39dc006501eb9d8ac0881357900548cf7)

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 <[email protected]>

(cherry picked from commit e91ad24b006823eadd6f3e21fc7cc5c8dd57b0d1)

(cherry picked from commit 3f08d2f986364315c3e43bde3524f12d069392ae)

…ache#6302)

(cherry-picked from 1de210b)

ExtraOperatorLinks are supported if Plugins are registered for them

(cherry picked from commit 9cb6e28)
(cherry picked from commit 72c75860ecfcd1930f1dedc7a0c713f122ea51a5)

(cherry picked from commit e840616)
(cherry picked from commit 6d01d8e5bac1b6e829b9da6fc50c1a4b6d23bcaf)

(cherry picked from commit 8a07aee3e5cf133c45ee4ae26aad6104c84502ab)

…ache#5874)

cherry-picked from apache@8f6ca53

)

(cherry picked from commit 3140c45)
(cherry picked from commit df65f8e)

(cherry picked from commit 99a5c2e)

This was incorrectly removed while cherry-picking and resolving conflicts