Add underscore equality operator to filter using keys in the request body

[laurenceisla]

WIP - Needs more discussion

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Using this idea https://github.com/PostgREST/postgrest/issues/2125#issuecomment-1200054135, the underscore operators will filter using the keys in the body. This PR implements the following:

• The _eq operator to verify for equality with items in the body
• Support to select any identifier in the body (not only PKs). This gives more freedom in the case of bulk updates, e.g.:

  PATCH /user?is_active=_eq.is_active HTTP/1.1
  
  [
    { "username": "userA", "is_active": true },
    { "username": "userB", "is_active": false }
  ]
  

• Full table updates are allowed once again, addressing this issue: https://github.com/PostgREST/postgrest/pull/2195#discussion_r932946412

[wolfgangwalther]

Wait, the comment in https://github.com/PostgREST/postgrest/issues/2125#issuecomment-1200054135 says:

Underscore operators could be used to operate on columns besides referencing values on the request body. This would allow doing https://github.com/PostgREST/postgrest/issues/1105#issuecomment-395238192 without computed columns:

SEARCH /PredictedGenes?Strand=eq.+&GeneStart=_gt.GeneEnd

Operating on a body value would still be allowed, body would be a reserved identifier:

SEARCH /PredictedGenes?Strand=_eq.body->strand&GeneStart=_lt.GeneEnd
{ "Strand": "-"}

With the implementation in this PR, the ability to do column to column matches will be lost, right?

[laurenceisla]

With the implementation in this PR, the ability to do column to column matches will be lost, right?

Yes. It was one of the reasons I turned this into a draft for now. It should use a syntax like ?col=_eq.$.col to get filters from the body and allow column matches too.