feat: Enable customization through a relationship filter

[mgagliardo91]

I had originally opened an issue, but thought it would be better to POC the change I was hoping for and see if this is something worth getting into the main branch.

The problem

We would like to use the pg-many-to-many plugin to enable relationships between linker tables. While the plugin does work for this, it also ends up creating a lot of relationships based on constraints across all the tables we have which result in very verbose resolver names, such as entityByEntityAIdAndEntityBIdAndEntityCId, etc.

The @omit manyToMany option does allow us to remove these resolvers, but because its opt-out first, we are having to do this on most constraints and it makes the growing schema difficult to manage, as it becomes a requirement that we can't forget to do.

Proposal

Allow consumers to pass a filter function that can let them decide when the derived relationship should result in a valid manyToMany connection. This function, here called pgManyToManyFilter, is backwards compatible and opt-in, so it has no downstream effects on existing usages.

An example of the usage, in the case of only wanting to enable for relationships where the linker table has a composite primary key, would look like:

...
graphileBuildOptions: {
    pgManyToManyFilter(
      leftTable: PostgraphileIntrospection,
      { junctionTable }: { junctionTable: PostgraphileIntrospection },
    ) {
      return (
        junctionTable.primaryKeyConstraint !== undefined &&
        junctionTable.primaryKeyConstraint.keyAttributes.length > 1
      )
    },
  },
...

I did not add any tests to validate this proposal, as I wasn't sure if it would be accepted as is, but I'd be happy to do so. We are currently consuming my fork, but I would prefer to get this change (or something similar) into the primary repo so we can point back to it.

Thanks again for contributing to OSS!