Added set_user methods for multi account user

[shunyooo]

Added a method to set the user for multi-account users, since user switching was not implemented.

from notion.client import NotionClient
client = NotionClient(token_v2=token_v2)

print(client.current_user.email)
# → not_the_desired@email.co.jp

client.set_user_by_email('desired@email.com')
print(client.current_user.email)
# → desired@email.com

[jamalex]

Great, thank you! If you have a chance, could you also add some instructions to the README?

[shunyooo]

@jamalex
Yes, with pleasure.
But I am not confident in my English, so please correct me if I am wrong in my sentences.
Thank you.

[HacDan]

@shunyooo Your English looks fine. For the sake of a cleaner commit log, I think it would make sense to add your README commit to this PR. I'm not a maintainer on this project, but that's how I would want it. Totally up to @jamalex