Expand OAuth2 coverage in README

[atc0005]

Overview

Some useful details to document for future reference:

• curl command to retrieve token from endpoint using client credentials grant
• expected access token response (JSON format)
  • clearly note that refresh tokens are not provided for client credentials grant
• describe the process of encoding the specified account/username/mailbox + access token
  • initial XOAuth2 string of "user=" {User} "^Aauth=Bearer " {Access Token} "^A^A"
    • originally handled by the emersion/go-sasl library, but later removed
      • now bundled internally
      • see Research: OATH2 support for accessing O365 mailboxes via IMAP #313 (comment)
    • handled by the https://github.com/Mailbutler/mail_xoauth2 Ruby gem
  • base64 encoding
    • handled by the emersion/go-imap Go library
    • handled by the Ruby stdlib Net::IMAP class
      • s = [data].pack("m0")
      • see Research: OATH2 support for accessing O365 mailboxes via IMAP #313 (comment)
• required scopes
  • setting them in Azure AD app registration
    • see Research: OATH2 support for accessing O365 mailboxes via IMAP #313 (comment)
  • specifying via tooling in access token request
    • https://outlook.office365.com/.default literal
    • https%3A%2F%2Foutlook.office365.com%2F.default urlencoded
• troubleshooting process of requesting token
  • link to https://github.com/DanijelkMSFT/ThisandThat/blob/main/Get-IMAPAccessToken.ps1 script
  • reference tool (not built yet) used to fetch token
    • see Add utility to obtain OAuth 2 token via Client Credentials flow #319
    • recommend running in debug mode

Not all of these would directly apply to tooling in this repo, but many would.

References

• Research: OATH2 support for accessing O365 mailboxes via IMAP #313
  • including the large volume of reference links collected thus far
• Add utility to obtain OAuth 2 token via Client Credentials flow #319
• https://github.com/DanijelkMSFT/ThisandThat/blob/main/Get-IMAPAccessToken.ps1