Refactor and add additional methods

[jdbcode]

Refactor:

The readme should briefly describe the options for connecting to EE and link to them (additional .md files, one for each method). Currently the readme only describes initializing by constructing google.oauth2.credentials.Credentials directly from environmental variable stored credentials (private key).

Methods:

• pytest-gee (includes instruction on connecting with private key and service account) and includes easy test file management
• Write private credentials secret to ~/.config/earthengine/credentials and pass result of ee.data.get_persistent_credentials() to ee.Initialize in scripts.
• [DONE] Write private credentials secret to environmental variable and ee.Initialize with google.oauth2.credentials.Credentials
• Use the three methods recommended by Google
  - (Preferred) Direct Workload Identity Federation
  - Workload Identity Federation through a Service Account
  - Service Account Key JSON

The last three should be emphasized, but if a service account or Workload Identity Federation are not possible for some reason, the private key could be used as a last less secure option.

cc @12rambau

[jdbcode]

This issue aggregates: #1, #3, #4