Refactor CLI to Command Pattern

[ryanmcdowell]

As the CLI has grown, there's many options that don't make sense to be specified together however they all exist for the same command (run). For example, --grouped-columns is an optional arg to the run command but it's only valid for a specific type of validation GroupedColumn. Same with --primary-keys and Row (#277).

To support future growth of the CLI and to simplify usage, I propose each validation becomes it's own top-level command. This would change the command-line from:

data-validation run [ARGS]

to

# Technically Column & Grouped Column is the same command with different opts 
# specified at runtime (e.g. the presence of a groupby key).
data-validation validate column [ARGS] 
data-validation validate schema [ARGS]
data-validation validate row [ARGS]

This change could be made backward compatible because the run command can continue to exist but now each command can be separated out into their own module with only the options relevant to that command present and inheriting from a validation command class (Command Design Pattern). This is similar to how we designed the gcloud SDK.