Extract extension methods from AstSchemaBuilder

[Axxiss]

As discussed #418 (comment) this PR aims to settle the ground for decoupling schema extension from materialization.

Creates a new trait AstSchemaExtender which contains all schema extension methods definition as originally defined in AstSchemaBuilder.

Furthermore, it creates a new default AstSchemaExtender implementation moving the implementation from DefaultAstSchemaBuilder to DefaultAstSchemaExtender

In order to keep backwards compatibility AstSchemaBuilder signature remains unchanged with the following tradeoffs

• AstSchemaBuilder extends AstSchemaExtender
• DefaultAstSchemaBuilder extends DefaultAstSchemaExtender

[travisbrown]

@Axxiss Would you want to make a case for this change going into 2.0? We're hoping to finalise what will be in it this week.

[Axxiss]

@travisbrown given the current situation with sangria. As this PR only moves code around without adding any value to end users, I think we can leave this out for now and wait for the next iteration.

If the new organization still want to move forward with #418 I will create the PR in the new repo.