[Core] az --version: Show command instruction and detailed instruction link when updates available

[fengzhou-msft]

Description

We can now rely on the AZ_INSTALLER env variable to know how the running Azure CLI is installed, so we can show specific instructions on how to update Azure CLI.

Example for Ubuntu:

You have 2 updates available. Consider updating your CLI installation with 'sudo apt-get update && sudo apt-get install --only-upgrade -y azure-cli'. Detailed instructions can be found at https://aka.ms/doc/UpdateAzureCliApt

For windows, for simplicity let's just recommend the MSI link for upgrading(The upgrade cmdlet on powershell needs to be run as Administrator and I'm still looking for the alternative to psutil to decide if CLI is running in Powershell):

You have 2 updates available. Consider updating your CLI installation with 'https://aka.ms/installazurecliwindows'. Detailed instructions can be found at https://aka.ms/doc/UpdateAzureCliMsi

For pip, 2 options for Linux and MacOS:
Linux/MacOs:

You have 2 updates available. Consider updating your CLI installation with 'pip install --upgrade azure-cli' or 'curl -L https://aka.ms/InstallAzureCli | bash' if you used our script for installing. Detailed instructions can be found at https://aka.ms/doc/UpdateAzureCliLinux

Windows:

You have 2 updates available. Consider updating your CLI installation with 'pip install --upgrade azure-cli'

Testing Guide

History Notes

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[yonzhan]

for upgrade

[yungezz]

just confirm, those short urls are all maintained actively

[fengzhou-msft]

all of them are active now.

[yungezz]

do we really need to print different messages based on platform? what about show all upgrade instructions?

[fengzhou-msft]

The current link shown is doing something similiar, it puts all the links to each package in one page. But we got complains about it's not being useful, some users may not know their operating system or packaging well, they have difficulties to find the right instruction.

[jiasli]

It would be helpful to show some rendered results in the PR description.

[jiasli]

Copy and paste a sample for other reviewers:

$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.2 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.2 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic