[PkgConfigDeps] pkg_config_custom_content property overwrites variables

[franramirez688]

Changelog: Feature: Let pkg_config_custom_content overwrite default *.pc variables created by PkgConfigDeps.
Changelog: Feature: Let pkg_config_custom_content be a dict-like object too.
Docs: conan-io/docs#3293

UPDATED:

• Simplified internal PkgConfigDeps logic: only one template and more straightforward, one context, Python is formatting the template, etc.
• pkg_config_custom_content overwrites Conan-defined freeform variables, not keyword metadata ones.
• pkg_config_custom_content could be defined as a dictionary instead of a string.