Introduce function ecm_install_configured_file

Summary:
This, as the name suggests, configures a file and installs it.

It's not very complicated but it's a repeated pattern in plasma that
gets quite messy dealing with temporary files.

Test Plan: Used in a project

Reviewers: Build System

Subscribers: apol, kossebau, pino, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D28355