Add `:constraint_handler` and `Ecto.Adapters.SQL.Constraint`

[petermueller]

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• [x] allows setting a :constraint_handler option in locations similar to :prefix
  • [x] config
  • [x] start_link
  • [x] per operation
• [x] adds Ecto.Adapters.SQL.Constraint to replace c:Ecto.Adapters.SQL.Connection.to_constraints/2
• [ ] moves existing to_constraints/2 to new modules for the built-in adapters
  • [x] Ecto.Adapters.MyXQL.Constraint
  • [ ] Ecto.Adapters.Postgres.Constraint
  • [ ] Ecto.Adapters.Tds.Constraint
• still calls the connection's to_constraints/2 for the time being to allow downstream adapters to adopt
• delegates the built-in adapter connections' to_constraints/2 to the new modules
  • e.g. Ecto.Adapters.MyXQL.Connection.to_constraints/2 -> Ecto.Adapters.MyXQL.Constraint.to_constraints/2
• [ ] adds tests for the built-in adapters verifying custom constraint handlers work
  • [x] MyXQL
  • [ ] Postgres
  • [ ] Tds
• [ ] adds documentation of how and when this feature is useful and how to use it