feat: Add region to access token

[liberty-rowland]

Feature

This PR adds AccessTokenOptions.region parameter to support regional authentication. This value will be encoded in the twr header of the JWT. If the parameter is not specified or invalid, no twr value is added. See this document for more details.
JIRA: https://issues.corp.twilio.com/browse/CLIENT-8142

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[eshanholtz]

Access Token docs need to be updated