[Catalog] Add logic for custom post launch actions (#6445)
[Catalog] Add logic for custom post launch actions (#6445)

## Related links
* Related bug: #5522
* Additional link: cl/229879820

## Introduction
In the future we may want to add custom launch actions to the application that will be performed at startup. This PR adds the ability to add an extension to `AppDelegate` that adds a function `performPostLaunchAction` to perform custom launch actions in the future if we need to.

## The problem
There is currently no hook to perform post launch actions within `MDCCatalog`.

## The fix
Add logic to check if a `Selector` exist and if so respond to it within `AppDelegate`. For additional context please see links above.

Closes #5522
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios