Add support for Verilator configuration (`.vlt`) files

### Description

I describe warning suppression in verilator configuration files
https://verilator.org/guide/latest/exe_verilator.html#configuration-files

Currently there is no way to include them in the openlane flow.

### Proposal

Add `VERILATOR_CONFIG_FILES` configuration variable, so the user can specify a list of configuration files.