Simplified file-retrieval support using HTSGET and SDA protocols

[mrtamm]

Overview of changes:

1. Added support for file-retrieval using protocol: sda://<dataset_id>/path/to/file
   • Bearer token is implicitly re-used from the "create-task" request.
   • The "create-task" request without a Bearer token will fail immediately.
   • It is possible to provide explicit Bearer token at the end of the URL after hash-sign; for example: sda://<dataset_id>/file#token-here
   • Supports downloading both Crypt4gh files (over /s3-encrypted/... path; decrypting *.c4gh files locally) and decrypted files (over /s3/... path)
   • Configuration just specifies the ServiceURL for the SDA to be used.
2. Simplified support for file-retrieval using HTSGET htsget://{reads|variants}/path/to/file-without-extension
   • Bearer token is implicitly re-used from the "create-task" request.
   • The "create-task" request will NOT fail without a Bearer token or credentials. (HTSGET may support non-authenticated requests.)
   • It is possible to provide explicit username:password or Bearer token at the end of the URL after hash-sign; for example: htsget://.../file#user:pass
   • Configuration just specifies the ServiceURL for the HTSGET to be used.
3. Refactored type names (simpler) in crypt4gh and htsget.
4. Updated documentation about HTSGET and SDA protocols.

[mrtamm]

PR has been updated with a fix for retrieving non-encrypted content from SDA.