Support per-table db-max-rows configuration via in-database settings

[raideno]

Problem

Limiting all database tables to a single max-rows value is quite restrictive. In practice, some tables are sensitive and should be queried in small quantities, while others can safely support larger result sets.

Solution

Allow db-max-rows to be configured per table, ideally via an in-database configuration mechanism.

[steve-chavez]

There are several shortcomings with db-max-rows:

• https://github.com/PostgREST/postgrest/issues/3576
• https://github.com/PostgREST/postgrest/issues/3651
• Clients don't know if it was applied https://github.com/PostgREST/postgrest/issues/2776

I've been thinking that db-max-rows really is part of a quota system. So taking https://github.com/PostgREST/postgrest/issues/4085 approach in mind, perhaps we can have a JWT like:

{
  "sub": "...",
  "iat": 1516239022
  "quota": {
    "max-rows": 1000
   }
}

This is global too, but in theory we could add table keys there (not sure if a good idea since JWT size could increase too much).

Advantages:

• max rows is still enforced
• max rows can be applied per application user, not just per role
• clients know they are being limited (the JWT can be decoded)
• we don't need to have a complicated setting inside the db or in postgREST config

Disadvantages:

• JWT size could increase too much, if we do support a per-table max rows.