[PP-1] The content:export command should allow you to specify which user account to use while exporting

Problem/Motivation

This is a follow-up to #3532694: Add a command-line utility to export content in YAML format.

Exporting content requires administrative privileges so that the system has the maximum possible access to all entity fields. Right now, it uses the AdminAccountSwitcher service to kick you into the first available account with administrative privileges. It would be better to let this be specified explicitly if desired; the default content importer allows this as well, so the exporter should too.

Proposed resolution

Add a --as-user|-u option to the content:export command. If passed, it needs to be the username or ID of an extant user account to use while exporting. If not specified, the current behavior is unchanged.