Webhook processor fixes

[lovromazgon]

Description

Followup to https://github.com/ConduitIO/conduit/pull/1552. The default from the deprecated field now conflicts with any headers.content-type option. This PR removes the default from the field and manually sets the default. It also updates an example to show how headers can be configured.

Additionally, it changes the behavior so a response status <500 is not considered a failing request.

Quick checks:

• [X] I have followed the Code Guidelines.
• [X] There is no other pull request for the same update/change.
• [X] I have written unit tests.
• [X] I have made sure that the PR is of reasonable size and can be easily reviewed.

[lovromazgon]

@raulb @hariso please have a look at the last commit, it allows the usage of Go templates in request.body.