Add getRefreshToken()

[psyntium]

For containers API, they require the refresh_token to be sent at the header. For some of its API.

Refer:
https://containers.cloud.ibm.com/global/swagger-global-api/#/clusters/GetClusterConfig

[csantanapr]

Thanks @psyntium for reporting the issue 👍

Wow, that's crazy, why do they want 2 tokens, sending the refresh token is only useful to get a new token if the primary token expired by the time they actually going to use it.

I welcome a PR with a backward-compatible change.
Maybe adding a plural version of the API functions getTokens and getHeathers that returns both tokens. and use header name X-Auth-Refresh-Token and default but allow override if there is another service that requires the refresh token with a different name,

[csantanapr]

Adding the function getRefreshToken also is useful I just notice now on the title of the issue

[psyntium]

   * This function sends a refresh token back through a Promise. The source of the token
   * is determined by the following logic:
   * 1. If the token is expired (that is, we already have one, but it is no longer valid, or about to time out), we
   *    load a new one
   * 2. If the token is not expired, we obviously have a valid token, so just resolve with it's value
   * 3. If we haven't got a token at all, but a loading is already in process, we wait for the loading promise to settle
   *    and depending on the result
   *    3a) use the newly returned and cached token
   *    3b) in case of error, trigger a fresh loading attempt
   * 4. If there is no token available and also no loading in progress, trigger the token loading
   *
   * @returns {Promise} - resolved with token value
   */
   getRefreshToken () {
    return new Promise((resolve, reject) => {
      const loadToken = () => {
        this.loadToken()
          .then(() => {
            resolve(this.tokenInfo.refresh_token)
          })
          .catch(error => reject(error))
      }

      if (this.isTokenExpired()) {
        // 1. load a new token
        loadToken()
      } else if (this.tokenInfo.refresh_token) {
        // 2. return the cached valid token
        resolve(this.tokenInfo.refresh_token)
      } else if (this.tokenLoadingPromise) {
        // 3. a token loading operation is already running
        this.tokenLoadingPromise
          .then(() => {
            // 3a) it was successful, so return the fresh token
            resolve(this.tokenInfo.refresh_token)
          })
          .catch(() => {
            // 3b) give it one more try - obviously, we hoped for a Promise triggered by another invocation to
            // return the token for us, but it didn't work out. So we need to trigger another attempt.
            loadToken()
          })
      } else {
        // 4. just trigger the token loading
        loadToken()
      }
    })
  }```