[7.x] Summary option for listing ingest pipelines without their definitions

[danhermann]

Addresses the need (mentioned here) for a compact listing of ingest pipelines without including their potentially long definitions by adding a summary option to the existing API so that this information is available in a stable format that is both machine and human-readable:

GET _ingest/pipeline?summary returns

{
  "my_pipeline_2" : { },
  "my_pipeline_3" : { },
  "my_pipeline_1" : { }
}

Relates to #31954

Backport of #69756

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)