Update API to work with token pairs

[slhr]

The api has been changed to work with a pair of tokens.

Before:
getToken: () => Promise<string>
saveToken: (token: string) => Promise<void>
refreshToken: () => Promise<string>

After:
getAccessToken: () => Promise<string>
saveTokens: (tokens: { accessToken: string, refreshToken: string }) => Promise<void>
refreshTokens: () => Promise<{ accessToken: string, refreshToken: string }>

Problem with the old approach:
When a 401 error occurred, the HTTP wrapper saved only one of the tokens, which caused issues when working with a pair.