Add special parameter in configuration for compression level

[bmironov]

Hello,

Thank you very much for your work on this great plugin.

I would like to propose little improvement. As we prefer to set for our pgBackRest backups compression levels higher that default value, it would be nice to have such setting along spec.confirguration.compression and not to hide it in additional parameters.

In my opinion something like spec.confirguration.compressionLevel for such purpose would be wonderful.

Thank you again and
Best regards,
Boris