{Core} Optimize performance for MSAL migration

[jiasli]

Description

As authentication logic is now split from Profile, performance can be further improved by lazy loading .auth.identity module.

> Measure-Command {python -m azure.cli version}
TotalMilliseconds : 384.9338

> Measure-Command {az version}
TotalMilliseconds : 404.2362

Imported modules can be inspected with

python -X importtime -m azure.cli version 2>perf.log; tuna .\perf.log

[jiasli]

This can eliminate the Azure Core import time:

python -X importtime -m azure.cli account get-access-token 2>perf.log; tuna .\perf.log

# Before
> Measure-Command {python -m azure.cli account get-access-token}
TotalMilliseconds : 1281.14

# After
> Measure-Command {python -m azure.cli account get-access-token}
TotalMilliseconds : 1070.6358

[yonzhan]

performance Optimization