Please provide a way to add licenses manually on oss-licenses-plugin

**Describe the bug**
This is related with https://github.com/google/play-services-plugins/issues/25
Several maven artifact does not provide license info on POM (such as retrofit, moshi and etc.. )
Also, several libraries can be included as non-maven artifact. (especially native libraries )

So it needs to add licenses manually, but oss-licenses-plugin does not allow.

**Additional context**
I made dirty modification to add licenses manually.
https://github.com/ganadist/play-services-plugins/tree/support_third_party_licenses

Just put license content under [`third_party_licenses` directory at root project](https://github.com/ganadist/play-services-plugins/blob/support_third_party_licenses/oss-licenses-plugin/src/main/groovy/com/google/android/gms/oss/licenses/plugin/OssLicensesPlugin.groovy#L40), and plugin will [generate license item with filename](https://github.com/ganadist/play-services-plugins/blob/support_third_party_licenses/oss-licenses-plugin/src/main/groovy/com/google/android/gms/oss/licenses/plugin/LicensesTask.groovy#L250-L253)