Add PJLink media player platform (#15083)

* add pjlink media player component

* retrieve pjlink device name from projector if name isn't specified in configuration

* update .coveragerc

* fix style

* add missing docstrings

* address PR comments from @MartinHjelmare

* fix code style

* use snake case string for source names

* add missing period at the end of comment string

* rewrite method as function

* revert to use source name provided by projector