Add credentials support

[yisraelU]

Add credentials support to Cache and fix auto-loading from config files

Expose credentials on the Java Cache API, matching the underlying Scala
FileCache model. Credentials stored in ~/.config/coursier/credentials.properties
(or COURSIER_CREDENTIALS) are now automatically picked up by all entry
points (Fetch, Complete, Versions, ArchiveCache, JvmManager). Also add the ability to manually add FileCredentials

Changes:

• Expand Credentials with host, realm, optional, matchHost, httpsOnly,
  and passOnRedirect fields to match coursier's DirectCredentials
• Add withCredentials/addCredentials/getCredentials to Cache
• Add addCredentials convenience methods to Fetch, Complete, and Versions
• Use addCredentials instead of withCredentials in ApiHelper.cache() so
  FileCache's default config-file credentials are preserved rather than
  overridden with an empty list
• Fix per-repo credential round-trip to preserve realm, optional,
  httpsOnly, and passOnRedirect through Authentication conversion
• Add comprehensive tests covering auto-loading, round-trips, all entry
  points, and the Credentials API surface

[alexarchambault]

Thanks for opening this, this looks great! I'll review that in more detail in the days to come, then merge I think.