Add input for receiving papertrail webhooks (#2038)

CHANGELOG.md

plugins/inputs/webhooks/README.md

plugins/inputs/webhooks/papertrail/README.md

plugins/inputs/webhooks/papertrail/papertrail_test.go

plugins/inputs/webhooks/papertrail/papertrail_webhooks.go

.../inputs/webhooks/papertrail/papertrail_webhooks_models.go

plugins/inputs/webhooks/webhooks.go

plugins/inputs/webhooks/webhooks_test.go