Check callable field sources before execution

[danbentley]

Firstly thanks for this package! It's helped remove a lot of glue code from a project!

I've spotted some GraphQL queries raising graphql.error.located_error.GraphQLLocatedError: 'str' object is not callable.

I tracked it down to custom_cls_resolver where a check is performed to see whether the block's class has an attribute with the same name as the queried value, to support the grapple.models.GraphQLField(source=<attr-or-method>) functionality.

The block I was querying contained a sub-block bound to name which during resolution, custom_cls_resolver would fetch the name attribute from wagtail.core.blocks.base.Block and attempt to execute it.

This PR adds a check to ensure that any attr is callable before attempting to execute it, allowing queries with blocks bound to name to resolve using the streamfield_resolver.

[zerolab]

Thank you for this, @danbentley

LGTM