Update request state usage to context variable

[wu-clan]

close #827

[downdawn]

使用starlette-context是不是会缺少IED提示，只能用get。
下面伪代码是否可以兼得

from starlette_context import context

class TypedContext:
    def __getattr__(self, key):
        return context.get(key)
    def __setattr__(self, key, value):
        context[key] = value

ctx = TypedContext()

ctx.user_id = "123"
print(ctx.user_id)

[wu-clan]

看起来很好，我会很快尝试一下