Re-consider access token lifespan

At the moment NSS defines a default 14-day token lifespan: https://github.com/solid/oidc-op/blob/main/src/AccessToken.js#L7
Why re-consider?

• a long life for NSS access tokens affects the entire Solid ecosystem. If another server (e.g. ESS, CSS) wants to support NSS access tokens, it needs to accept tokens with a very long life.
• Bearer tokens (which is what we're talking about) are global in nature for Solid. They are extremely powerful and so any exfiltration can have significant negative consequences (i.e. a user loses data to a malicious actor). Reducing the lifespan of a global access token reduces risk for everyone
• for access tokens (especially global access tokens) the industry standard is <= 1 hour. For access tokens that secure "sensitive" data, the standard is closer to 10 minutes or less

Suggestions:

1. Request a refresh token during the authorization code flow (this is part of OpenID Connect). Then, when the token is nearing expiry, exchange the refresh token for a new access token
2. Completely refresh the app, passing the user through the full authorization code flow (part of OIDC). There will still be a cookie available on the identity provider, so the user will, in all likelihood, be passed right through the redirect flow and end up where they started. There are patterns for maintaining state between the start and end of that flow

• If the cookie expired on the Identity Provider, then the user would need to re-enter credentials
• NSS uses cookies so we could exchange the access token with a cookie.
  Example: Gmail uses cookies. Cookies are easy to scope to a particular app, so there are no significant security issues.

从 src/AccessToken.js 的默认 14 天有效期开始，然后审查 issue 中描述的 authorization code flow、refresh-token option、cookie 处理和 OpenID Connect 注意事项。实现前需要确定安全设计；done 应包括一项已达成共识的 token 生命周期策略，以及该策略对续期和重新认证的影响。