`az account get-access-token`: Use epoch `expiresOn`/`expires_on`

[jiasli]

Context

Currently, the expiresOn property in the output of az account get-access-token is a string representing the local time:

{
  "accessToken": "eyJ0eXAiOiJ...",
  "expiresOn": "2021-09-27 16:16:30.921462",  <<<
  "subscription": "0b1f6471-1bf0-4dda-aec3-cb9272f09590",
  "tenant": "54826b22-38d6-4fb2-bad9-b7b93a3e9c5a",
  "tokenType": "Bearer"
}

This inherits from the ADAL behavior: https://github.com/AzureAD/azure-activedirectory-library-for-python/blob/35f9e003bed47936a1df3c6eb696691dc1fa91c3/adal/oauth2_client.py#L187

This causes various time computation problems and difficulties, especially for daylight saving time, such as

• Correct expires_on values of tokens from AzureCliCredential azure-sdk-for-python#14331
• PEP 495 -- Local Time Disambiguation: https://www.python.org/dev/peps/pep-0495/

Other tools or services are already using an integer to represent the epoch time:

• Azure Python SDK uses integer expires_on: https://github.com/Azure/azure-sdk-for-python/blob/844e16db0abc553ab1adf104128cbf0e223af189/sdk/core/azure-core/azure/core/credentials.py#L14
• MSAL internally uses integer expires_on: https://github.com/AzureAD/microsoft-authentication-library-for-python/blob/db6f001060e0adcc1edccac1a86ec05bd59f15f0/msal/token_cache.py#L176
• Managed identity's result uses integer expires_on:

  azure-cli/src/azure-cli-core/azure/cli/core/adal_authentication.py

  Line 116 in 14cc787

  # "expires_on": "1605686811",

This approach is universal and eliminates all necessary complexity.

Proposal

There are possible approaches:

1. Make a BREAKING CHANGE to convert the expiresOn in the output of az account get-access-token to an integer representing the epoch time. This can be done during MSAL migration: ADAL to MSAL migration #18944
2. Introduce another property expires_on. This will not be a BREAKING CHANGE, but introduces naming inconsistency as other properties uses camelCase, but expires_on is snake_case.
3. Introduce another properties like expiresOnEpoch, but naming like this is non-conventional.