[Feature Request] Define default configuration from environment variable

**Is your feature request related to a problem? Please describe.**
Unable to define default configuration for current process context only.
This is especially useful when running on an Azure build agent which might be running multiple pipelines concurrently.

**Describe the solution you'd like**
Support defining default configuration from environment, [as supported by Azure CLI](https://docs.microsoft.com/en-us/cli/azure/azure-cli-configuration#cli-configuration-values-and-environment-variables).
Experience _should_ looks like:
1. `$ az login`
2. `$ export AZURE_DEVOPS_EXT_DEFAULTS_ORGANIZATION=https://dev.azure.com/myorg`
3. `$ az repos list --project myproject --detect false`

**Additional context**
Originally reported as a bug in #1121, but turned out to be a feature request according to latest comment.