Deprecate context.add function

[ThisaruGuruge]

Description:
In the graphql:Context object, there is an add function to add attributes to the context. But as per the convention, (similar to http package), the function name should be set instead of add.

Since this change will be a breaking change, the add function have to be deprecated and the set function should be introduced. In a later release, the add function can be removed.