"rename_graphql_field()" implemented and tested.

[kidunot89]

What does this implement/fix? Explain your changes.

• rename_graphql_field( $type, $field_name, $new_field_name ) implemented.
  • Use to replace conflicting field name.
• AccessFunctionTest updated to WPGraphQLTestCase

Any other comments?

This allows users to replace a field name on a Type.

For example, let's say we want to rename the root user field to wpUser.

We could do that like so:

add_action( 'graphql_register_types', function() { 
  rename_graphql_field( 'RootQuery', 'user', 'wpUser' );
} );

And now we can see the wpUser field in the Schema, instead of the user field.

[coveralls]

Coverage remained the same at 78.928% when pulling ef2a02c on kidunot89:feature/replace-graphql-field-name into 4c47420 on wp-graphql:develop.