Make gql_code_gen:ast_builder extension configurable

[micimize]

Would be good to expose an extension: '.gql' option

gql/gql_code_gen/lib/ast_builder.dart

Line 40 in 38805d3

".graphql": [astExtension],

[klavs]

@micimize, I'm lacking in-depth knowledge about dart build process. I think I read that the extension is not configurable. I may be wrong. Do you have any reference to docs or any other project where a similar configuration is possible?

[micimize]

hmm, actually you might be right. I guess in artemis they just
have a separate glob

final assetStream = buildStep.findAssets(Glob(schemaMap.queriesGlob));

[klavs]

I'm open to adding .gql as the other default extension.