refactor(js, client): refresh token should be optional in RefreshToke…

[charIeszhao]

…nTokenResponse type

Summary

According to RFC6749, the auth server MAY issue a refresh token, not always.

Testing

Checklist

• [x] .changeset
• [x] unit tests - [ ] integration tests
• [x] Updated SDK convention docs

OR

- [ ] This PR is not applicable for the checklist

[linear[bot]]

LOG-6857 Update refresh token response type in JS SDKs

According to [RFC6749](https://datatracker.ietf.org/doc/html/rfc6749#section-6), the server MAY issue a refresh token, not always.