[Config] Make config items case-insensitive

[evelyn-ys]

Description

Fix https://github.com/Azure/azure-cli/issues/14397: az configure --list-defaults should not be case sensitive for environment variables

Testing Guide

preparation(need Azure CLI in advance)

az configure --defaults location=eastus
export AZURE_DEFAULTS_LOCATION=eastasia

test

az configure --list-defaults

results before

[
  {
    "name": "LOCATION",
    "source": "AZURE_DEFAULTS_LOCATION",
    "value": "eastasia"
  },
  {
    "name": "location",
    "source": "C:\\Users\\yishiwang\\.azure\\config",
    "value": "eastus"
  }
]

results after

[
  {
    "name": "LOCATION",
    "source": "AZURE_DEFAULTS_LOCATION",
    "value": "eastasia"
  }
]

[evelyn-ys]

looks good now