Support for protoc 3.15.0

[programmatix]

First, thank you for this awesome tool.

We'd like to use the support for optional params added recently in protoc 3.15.0. Do you have any plans to add embedded support for this release?

[KeatsPeeks]

@programmatix As a workaround, you can use <protocVersion>3.15.0</protocVersion>. It will download protoc from the central Maven repository.

You might have to configure your <home>/.m2/settings.xml correctly if you're behind a proxy or using a company mirror.

[os72]

This tool is looking for support in the form of sponsorship (most users are enterprises)..

[programmatix]

Thanks for the suggestion @KeatsPeeks. After also upgrading everything to latest, it worked great.