Rework output format of pg_dependenciesHEADmaster

The existing format of pg_dependencies uses a single-object JSON
structure, with each key value embedding all the knowledge about the
set attributes tracked, like:
{"1 => 5": 1.000000, "5 => 1": 0.423130}

While this is a very compact format, it is confusing to read and it is
difficult to manipulate the values within the object, particularly when
tracking multiple attributes.

The new output format introduced in this commit is a JSON array of
objects, with:
- A key named "degree", with a float value.
- A key named "attributes", with an array of attribute numbers.
- A key named "dependency", with an attribute number.

The values use the same underlying type as previously when printed, with
a new output format that shows now as follows:
[{"degree": 1.000000, "attributes": [1], "dependency": 5},
 {"degree": 0.423130, "attributes": [5], "dependency": 1}]

This new format will become handy for a follow-up set of changes, so as
it becomes possible to inject extended statistics rather than require an
ANALYZE, like in a dump/restore sequence or after pg_upgrade on a new
cluster.

This format has been suggested by Tomas Vondra.  The key names are
defined in the header introduced by 1f927cce4498, to ease the
integration of frontend-specific changes that are still under
discussion.  (Again a personal note: if anybody comes up with better
name for the keys, of course feel free.)

The bulk of the changes come from the regression tests, where
jsonb_pretty() is now used to make the outputs generated easier to
parse.

Author: Corey Huinker <[email protected]>
Reviewed-by: Jian He <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com

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