Content-app doesn't respect response-headers specified by plugins

[ggainey]

Version
3.28

Describe the bug
core/3.28 added Distribution.content_headers_for() to allow plugins to specifiy response-headers. When using non-FileStorage backends, these headers are ignored.

pulp_rpm PR pulp/pulp_rpm#3193 , for example, fails on azure and S3 because the cache-disposition header isn't ending up in the response.

To Reproduce

• start with core/3.28 using a minio-storage-backend
• install pulp_rpm with Take advantage of Dist.content_headers_for() to no-cache repomd and f… pulp_rpm#3193
• create, sync, and distribute a pulp_rpm repo
• wget the repomd.xml file from that distribution
• note the absence of a cache-control header.
• be sad.

Expected behavior
headers added by a plugin should be made available in the response

Additional context
With azure and google, looks like we can just feed the headers into response-parameters. S3 may need specific code.