Support using pre-hashed passwords

Asyncpg should also support using HASH objects or pre-hashed strings. Also, I would suggest that once the authentication method has been identified (e.g. md5, sha, no-hash) then the password should be morphed. As it currently stands, the password can be retrieved in plain text by other libraries that have access to the Connection or Pool objects.

# malicious code can easily get away with this:
print(pool._connection_kwargs['password'])

首先追踪 asyncpg 的 Connection 和 Pool 如何处理密码，以及如何识别 md5、sha 和 no-hash 等 PostgreSQL 身份验证方法。完成的标准是支持预先哈希的字符串或 HASH 对象，并且密码处理不再通过 pool._connection_kwargs 暴露明文值。