Implement __type/__schema introspection disabling

[TomasBarry]

A question was asked in issue GH-2546 asking whether it was possible to
disable a single introspection entry point considering that #2327 had
implemented then functionality to disable introspection as a whole using
disable_introspection_entry_points. In the issue, an option was laid
out to implement the functionality for individual
disable_schema_introspection_entry_point and
disable_type_introspection_entry_point options. This commit implements
these methods.

These new methods are used in an identical manner to
disable_introspection_entry_points:

Disabling __type introspection

class MySchema < GraphQL::Schema
  disable_type_introspection_entry_point
end

With the above, attempting to execute a query with __type
introspection will return the following error message:

Field '__type' doesn't exist on type 'Query'

Disabling __schema introspection

class MySchema < GraphQL::Schema
  disable_schema_introspection_entry_point
end

With the above, attempting to execute a query with __schema
introspection will return the following error message:

Field '__schema' doesn't exist on type 'Query'

Note, having both disable_type_introspection_entry_point and
disable_schema_introspection_entry_point is identical to having just
disable_introspection_entry_points. Having
disable_introspection_entry_points and
disable_type_introspection_entry_point or
disable_schema_introspection_entry_point is no different to just
having disable_introspection_entry_points.

[rmosolgo]

Thanks for your work here! It looks great. I just pushed some changes to address the lint errors, then I'll merge it.

[TomasBarry]

For some reason I did not spot those errors. Thank you.