Configurable s3 endpoint for download

[aaperis]

Related issue(s) and PR(s)
This PR closes #750.

Description
With this PR download can either be deployed as serving either unencrypted or encrypted files. The serveUnencryptedData boolean is removed as well as the logic for download to always generate an internal c4gh key-pair (that would allow the service to serve unencrypted files in case serveUnencryptedData=true).

Main changes:

• download can now be configured to only serve unencrypted files if a filepath to a c4gh private key file is provided. Internally, the code checks the validity of the provided key by using the supplied passphrase to retrieve the corresponding c4gh public key. The service will not start if this procedure fails and the service will serve unencrypted data only if the retrieved public key is non-empty. This ensures that there can be no misconfiguration by e.g. providing malfunctioning keys etc. For example, the crypt4gh library and therefore reencrypt will use any string provided to it and encrypt a file but with this mechanism we avoid such a scary scenario.
• if no filepath and passphrase are provided, then download will serve only encrypted files. This is the default behavior.

Other changes include:

• go testsuite updates and some refactoring along the way
• update of dev environment: in order to be able to test both setups at once, the docker compose files have been modified to include two instances of download listening at different ports, one serving encrypted and the other serving unencrypted files
• update of integration tests, including a separate bash script for error messages returned to the user (according to the issue description)
• update of helm chart

How to test
Integration tests pass.

[jbygdell]

Crypt4gh config already exists, there is not need to add another one since the app will only need one.

Either reuse the existing one or move everything up under the app sub heading.

[aaperis]

I unresolved the initial the conversations that I set as resolved (these are answered) so that @jbygdell can take a look and resolve. According to how we decided to do things on our last meeting about PRs.

[MalinAhlberg]

Looks like good and thorough work..!
I left some questions, but otherwise I think it looks good!