Add get user functionality

[codemation]

Description

This PR adds get_user() functionality to easyauth decorated endpoints.

Example

from fastapi import FastAPI

from easyauth.client import EasyAuthClient
from easyauth import get_user

server = FastAPI()

@server.on_event('startup')
async def startup():
    server.auth = await EasyAuthClient.create(
        server,
        token_server='0.0.0.0',
        token_server_port=8090,
        auth_secret='abcd1234',
        default_permissions={'groups': ['users']}
    )

    # grants access to users matching default_permissions
    @server.auth.get('/default')
    async def default(user: str = get_user()):
        return f"{user} is accessing default endpoint"