clarify Okta and AWS session token durations / expiries

[monde]

okta-aws-cli needs to clarify in documentation and code:

(1) Document how the three token types that occur are used during operation

1. Access token from Okta OIDC app
2. Session token from Okta AWS Federation app
3. AWS session token

(2) Document Okta OIDC app access token expiry can not be set and is always 60 minutes

(3) Document --cache-access-token applies to that Okta OIDC app's access token and is written to ~/.okta/awscli-access-token.json

(4) Document how okta-aws-cli takes the Okta OIDC app's access token and presents it to the Okta AWS Federation app, and in turn the Okta AWS Fed app fetches a SAML assertion from AWS given a valid access token.

(5) Document --session-duration applies to the AWS Session Token and it has a valid range of minimum 900 seconds (15 minutes), max 1 to 12 hours depending on AWS settings see https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_manage-assume.html

(6) Improve documentation that --write-aws-credentials writes IAM creds to ~/.aws/credentials

(7) Add better input validation and error messages around AWS session duration parameter.