Add a JF_PLUGINS_REPO to complement JF_DEPS_REPO for Maven builds

Is your feature request related to a problem? Please describe.
We configure Artifactory and Maven to use different Maven repositories for plugins and dependencies. Running Frogbot this way is failing because it's ignoring Maven settings files and subsequently can't load the plugins.

Describe the solution you'd like to see
I would like to see Frogbot just use the already-correct Maven configuration directly instead of have to perform redundant configuration that developers will lose a day to when they forget they have to change Frogbot configuration every time they change the Maven configuration. There is no implementation of frogbot that would make developer adoption easier and faster than just directly supporting the Maven configuration. But barring that I would like to see a JF_PLUGINS_REPO env variable so that we can maintain our separation of plugins vs dependencies.

Describe alternatives you've considered
After talking with JFrog support it seems our only option is to combine our plugins and dependencies Maven repos with virtual repos in Artifactory and configure JF_DEPS_REPO with that combined repo.

The issue names no files or tests, so begin by tracing Frogbot's Maven repository configuration and its environment-variable handling. Done means Maven plugin resolution can use a separate JF_PLUGINS_REPO while dependency resolution continues using JF_DEPS_REPO; the issue also raises direct Maven settings support as an alternative to investigate.