chg: Set up the main running function to use handler functions

[cryptaliagy]

A very small PR with no associated issue because it's just a little thing I noticed.

While we do enable users and plugins to add options to the parser, one use case that is currently not properly supported is to use subparsers to add functionality to Quilla that is not directly related to validations. For example, if a developer would like to add a command such as quilla az that enables users to download the images from their azure blob storage, their plugin would not actually require running any validations.

In these cases, the plugin is adding functionality that is intimately coupled with Quilla, but the proper execution flow of Quilla should be interrupted to enable this. To that affect, this PR addresses this need by using the concept of "handler functions"- Default functions that are set at the parser level which dictate the "main" execution flow after the Context object is created.

Going back to the blob storage download plugin, a plugin author could create a subparser for their plugin that allows users to have the command quilla az download <run_id>, and set the subparser's handler function to be one of their own. Then, when Quilla calls ctx.run(), the subparser function that was specified would be called whenever a user calls quilla az download <run_id>

[cryptaliagy]

Changing this PR to a draft because I might need to change up some of how the CLI options are parsed to allow this to work effectively and I don't know if it makes sense to do it at this point

[daniellopezgarcia]

LGTM!