[FEATURE] Read Sonatype Guide credentials from a local dotfile

[bhamail]

The yaml format in which we store credentials has changed in Nancy v1.0.0. Jake should be updated to be able to read the same files so such credentials can be shared across the tools.

cc @bhamail / @DarthHater

[madpah]

Jake v4.0.0 has moved away from OSS Index to Sonatype Guide, so this issue no longer applies to a Nancy-compatible credentials file format. However, the underlying intent — sharing credentials conveniently across tools — remains valid.

Updating this issue accordingly: the question becomes whether Jake should support reading SONATYPE_GUIDE_USERNAME / SONATYPE_GUIDE_TOKEN from a local dotfile (e.g. ~/.sonatype/credentials) in addition to environment variables, for parity with other Sonatype tooling.