[feature request] Support for authorization via username/password

[raymond-ti]

When Nifi has authentication enabled, using this plugin will be failed because access token is not passed when calling Nifi APIs. If config can be extended to include username and password, then call POST /access/token to get the access token, then pass the returned token as Authorization header when call Nifi APIs. The token lifetime lasts for 12 hours, token refresh handling may not be needed.

[egorpushkin]

I believe this request will be addressed by the work done here: #8
It is currently in review and will be merged soon.

[egorpushkin]

As it turns out we are going to get support for client cert authentication but not for username/password authentication just yet. This needs to be added separately.