Provide an "effective" merged devcontainer.json inclusive of Features from read-configuration command

[Chuxel]

Currently the devcontainers read-configuration command has two modes. One that shows the contents of devcontainer.json and one that also shows the feature configuration for any features referenced (via the --include-features-configuration).

However, what would be most useful would be a way to get a merged configuration inclusive of both out of the CLI. For example, adding a --effective-configuration flag:

$ devcontainer read-configuration --effective-configuration true --workspace-folder .

This would be extremely useful for tool or service implementors that need to any pre-processing steps prior to starting a dev container, or need to inject contents based on the configuration.

As a concrete example, we could use this to modify the tools-vscode-server example under example-usage in this repository to adapt to customizations.vscode.settings and customizations.vscode.extensions through the use of jq and some shell scripts. Right now you'd have to manually parse each feature and do a merge yourself - which could deviate from the standard logic.

[Chuxel]

Looks like --include-merged-configuration does exactly this!