Extend ability to capture HTTP headers to cover the same scope as the ELASTIC_APM_CAPTURE_HEADERS configuration option of classic agent does.

Ability to sanitize data should be covered separately by #307

For Java this should already be supported for HTTP request headers (and maybe responses), and the main gap is for the messaging systems:

• JMS
• SQS
• RabbitMQ
• not supported for Kafka, but would be worth covering due to popularity