Add HTTP security headers

**Issue by [pawelprazak](https://github.com/pawelprazak)**
_Monday Apr 18, 2016 at 14:58 GMT_
_Originally opened as https://github.com/akka/akka/issues/20357_

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[OWASP](https://www.owasp.org/index.php/List_of_useful_HTTP_headers) lists the most common security related headers:
- X-Frame-Options
- X-Content-Type-Options
- X-XSS-Protection
- Content-Security-Policy
- Public-Key-Pins
- Strict-Transport-Security

Do you plan to add them, or is it out of scope?

If I would to make a pull request, is this the right place to start:
- `akka.http.scaladsl.model.headers`
- `akka.http.javadsl.model.headers`
- `akka.http.impl.model.parser`

Start by reading akka.http.scaladsl.model.headers and akka.http.javadsl.model.headers, then inspect akka.http.impl.model.parser to understand how the listed HTTP headers are represented and parsed. Done means the requested OWASP security headers have a defined, usable path through the relevant HTTP model APIs and parser, with the scope of supported headers resolved.

Written by the indexing model from the issue text.