Ensure that the preset.grafast.context.pgSettings example in the docs actually works

[benjie]

Have a vague feeling you need to define it via the pgServices section instead due to an assertion that prevents overwriting. We should check and either remove the assertion or fix the docs.

declare global {
  namespace Grafast {
    interface Context {
      user?: unknown
      snowflakeConnection?: unknown
    }
  }
}
const preset: GraphileConfig.Preset = {
  grafast: {
    explain: true,
    context(ctx) {
      const user = ctx.node?.req.user;
      return {
        user,
        snowflakeConnection: ctx.node?.req.snowflakeConnection
        pgSettings: {
          ...args.contextValue?.pgSettings,
          "jwt.claims.user_id": user?.id,
        }
      }
    },
  },

^ Note this is probably invalid currently - I think you actually have to configure the pgSettings via pgServices: [makePgService({pgSettings(ctx){return {'jwt.claims.user_id': ctx.node?.req.user?.id}}}) call due to an over-cautious protection to prevent you accidentally overwriting pgSettings.