proposal: allow the custom template for server code generation

[OldBigBuddha]

Overview

I have made gqlgen possible to set custom templates for the server code generation that I previously proposed at #3371.

If you set the file or directory path to the template in config as below, code generation will be performed by referring to those files rather than the templates managed by gqlgen:

# Where are all the schema files located? globs are supported eg  src/**/*.graphqls
schema:
  - graph/schema/**/*.graphqls

# Where should the generated server code go?
exec:
  filename: graph/generated/autogen.go
  package: generated
  exec_template_dir: graph/template
  # you can specify a single template file
  # exec_template: graph/template/exec.gotpl

This change allows users who want to change the core logic of the server for exceptional use cases to continue to use the original gqlgen with only custom template preparation.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 60.584% (+1.0%) from 59.549%
when pulling d6e2037 on OldBigBuddha:patch
into aaf44f5 on 99designs:master.

[StevenACoffman]

While this is excellent work, and is a very useful example for people who want to experiment with alternative executions, I'm going to close this PR given that we found a less invasive way to provide people with control over the trade off between memory and speed.