Add oauth2 configuration (2/2)

[aprodhomme-dkt]

This PR comes after having reviewed and merged the PR #171

It provides an optional configuration to use with API publishers that use OAUTH2 authentication but with their own form fields.
For example requesting an access token with the following form (eg: https://api.didomi.io/docs/#/sessions/post_sessions):

{
  "type": "api-key",
  "key": "my_client_id",
  "secret": "my_secret"
}

We would have the following configuration:

{
  "name": "http-sink-test",
  "config": {
    "connector.class": "io.aiven.kafka.connect.http.HttpSinkConnector",
    "topics.regex": "domain_a.*",
    "http.url": "http://httpmockserver:1080",
    "http.authorization.type": "oauth2",
    "http.headers.content.type":  "application/json",
    "oauth2.access.token.url":  "http://httpmockserver:1080/oauth2",
    "oauth2.client.authorization.mode": "URL",
    "oauth2.grant_type.key": "type",
    "oauth2.grant_type": " api-key",
    "oauth2.client.id":   "my_client_id",
    "oauth2.client.id.key":  "key",
    "oauth2.client.secret":  "my_secret",
    "oauth2.client.secret.key":  "secret",
    "key.converter": "org.apache.kafka.connect.storage.StringConverter",
    "value.converter": "org.apache.kafka.connect.json.JsonConverter"
  }
}

[jeqo]

Some comments on the main implementation. Will look into the tests a bit later.

Thanks @aprodhomme-dkt !

[jeqo]

@aprodhomme-dkt could you rebase changes from main, so we can review commits only for this change.

ps. forgot to squash commits from #171, so main history got messed up a bit.

[aprodhomme-dkt]

@aprodhomme-dkt could you rebase changes from main, so we can review commits only for this change.

ps. forgot to squash commits from #171, so main history got messed up a bit.

done.

[jeqo]

Commits seem a bit strange still. It should only have the last 5 included in your PR, but there are 21.

[aprodhomme-dkt]

Commits seem a bit strange still. It should only have the last 5 included in your PR, but there are 21.

indeed. I'll try to make it clearer. I might have to push force.

[aprodhomme-dkt]

I think you're good to go now @jeqo ;)

[jeqo]

@aprodhomme-dkt Looking good! just a few minor comments and validation improvement suggestions.

[jeqo]

@aprodhomme-dkt sorry for the delay. Hope to look into this later this week. Thanks.

[jeqo]

@aprodhomme-dkt thanks for looking into the suggestions! I think there's only one missing and it should be good to go.

[jeqo]

LGTM, thanks @aprodhomme-dkt for your contribution!