fix(downloads): set content-type and content-length headers

[andrewazores]

Fixes #705

This improves the user experience when requesting files for download, particularly through the web-client. Content-Type and Content-Length headers are set where they were previously missing on some GET handlers, so that requesting clients are always provided with this information about the requested asset.

[jan-law]

For GET recording download, is the [16297 bytes data] the octet-stream equivalent of a content-length header?

> GET /api/v1/targets/service:jmx:rmi:%2F%2F%2Fjndi%2Frmi:%2F%2Flocalhost:0%2Fjmxrmi/recordings/test HTTP/1.1
> Host: 0.0.0.0:8181
> User-Agent: curl/7.71.1
> Accept: */*
> 
{ [5 bytes data]
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< transfer-encoding: chunked
< content-type: application/octet-stream
< 
{ [16297 bytes data]
100 1229k    0 1229k    0     0  6274k      0 --:--:-- --:--:-- --:--:-- 6306k

GET report for comparison:

> GET /api/v1/targets/service:jmx:rmi:%2F%2F%2Fjndi%2Frmi:%2F%2Flocalhost:0%2Fjmxrmi/reports/test HTTP/1.1
> Host: 0.0.0.0:8181
> User-Agent: curl/7.71.1
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< content-type: text/html
< content-length: 120901

[andrewazores]

GET on a report can always provide a Content-Length since once the report is generated as HTML, we always know exactly how large that document is and can report it in the header.

GET on an archived recording can also have a Content-Length, because we have that file on disk and can measure its size when we send it.

GET on an active recording has no Content-Length because Cryostat doesn't know how large the recording is - it just reads it from the remote JVM and pipes it through to the requesting client as a stream. At the time that it sends the response headers, the length is unknown. The transfer-encoding is set to chunked to reflect this streaming nature.

[jan-law]

Looks good to me! I'll leave it to @ebaron for final review comments/approval

[ebaron]

Looks good!