fix: Use jwtDecode to parse the token

[Vitor-Avila]

An atob error could was being logged in the console depending on the payload used to generate the guest token. Example problematic payload:

{
    "user": {"username": "[email protected]", "first_name": "Vitor", "last_name": "Avila"},
    "resources": [{"type": "dashboard", "id": "${DASHBOARD_ID}"}],
    "rls": [
        {"clause": "user_visibility @> ARRAY['[email protected]']"},
    ],
}

Error faced:

Uncaught (in promise) InvalidCharacterError: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.

atob does not handle base64url encoded tokens. This PR updates the implementation to use jwt-decode (same approach as Superset OSS: https://github.com/apache/superset/blob/master/superset-embedded-sdk/src/guestTokenRefresh.ts#L19-L27