Add extensions support

[Fontinalis]

I added an Extension interface that enables to build extensions for the graphql package.
Using that, we can add support for the extensions and add tracing as an extension.

Added an example examples/star-wars-with-trace that shows how we could add a tracing extension.

Using the interface we could add different extensions based on different tracing providers.

I'm interested in any kind of feedback, happy to work on this more.

Update 1:
Removed the example from the repo, and created one here
Fontinalis/graphql-tracing-example

[coveralls]

Coverage increased (+0.2%) to 92.338% when pulling b92b14d on Fontinalis:extensions into cc858c0 on graphql-go:master.

[Fontinalis]

FYI there’s an example at graphql-tracing-example which is a simple opentracing example based on this PR

[Fontinalis]

@chris-ramon Any update on when this will be merged or reviewed?

[Fontinalis]

@chris-ramon Any luck with looking into this and make some feedback on the PR?

[chris-ramon]

@Fontinalis thanks a lot for working on this 👍 💯

I've being following the progress and this looks really great to me, the extensions API is small and simple to use.

Also thanks for putting altogether and creating a neat example:

Merging this one and let's address further improvements in a follow-up PRs.

[zonr]

My two cents:

1. "Extension" may not be a good name under the context of Schema because it may cause confusion with Schema Extension defined in GraphQL specification.

• However, graphql-ruby uses the name FieldExtension.
• Both Sangria and graphql-dotnet refers the feature as Middleware.

2. graphql-js plans to add middleware ([Feature Request] Resolver Middleware graphql/graphql-js#1516) in v15 to allow extended behaviors to be added to resolvers and schema.

• But I guess they may not have that many hooks as in this PR such as ParseDidStart and ValidationDidStart, etc..

3. The design currently binds Extensions to Schema. What do you think to pass it as part of ExecuteParams?

In any way, many thanks again for your hard work to bring this feature!