<!-- The PR description should answer 2 important questions: -->

### What

Operator mappings in boolean expressions allow us to override the name
of an operator, so we can call some database's `_eq` operator the much
more attractive `equals`. If not override is provided we can just use
the database's name directly. We make this a newtype with a `get`
function to encode this logic and make doing the right thing the easiest
thing.

V3_GIT_ORIGIN_REV_ID: e47f4e291494562ed1551d0d7469259198862a0a

<!-- The PR description should answer 2 important questions: -->

### What

Previously the OpenDD pipeline has ignored `BooleanExpressionType` as
they were not available outside GraphQL. Now they are, so we use them,
and test them in the JSONAPI pipeline.

This means that the SQL frontend cannot use filtering unless models have
a `BooleanExpressionType` defined. These are generated by the CLI, so
hopefully this is fine.

Depends on #1336

V3_GIT_ORIGIN_REV_ID: 4037585ef36bc99c99a482361084ccd09c5c6bb1