Allow `-D` options to be passed to forked spring boot

[cah-nathan-zender]

I have an application that requires spring profiles to be set to run locally. This means that when I try to run the task when it spins up the app but of course it does not work.

What i run now
./gradlew generateOpenApiDocs

Suggestions

Allow passing in them from whatever is passed to gradle
./gradlew generateOpenApiDocs -Dspring.profiles.active=localdb,otherprofile,etc

Although this might also requires configuration at the task level...see here
https://mrhaki.blogspot.com/2015/09/gradle-goodness-pass-java-system.html

So maybe making it a configuration option makes it easier/more straight forward.

openApi {
    forkedSystemProperties = "-Dspring.profiles.active=localdb,otherprofile,etc"
}

I'm willing to help with a PR just would be interested in feedback on what direction feels like the better option.

[zendern]

Was able to pretty easily get both options originally mentioned in PR #23

[bnasslahsen]

v1.3.0 is now released and integrates the proposed PR from @zendern.