Allow configuring the target name for a sample directory

After #6 is merged, there is still one use-case missing for replacing the [Sample Rule](https://github.com/gradle/gradle/blob/8a7b8ea7bfd2c38689c8fa339c66485f610c10a9/subprojects/internal-integ-testing/src/main/groovy/org/gradle/integtests/fixtures/Sample.java) in `gradle/gradle`: The `Sample` rule allows configuring `testSampleDirName`, the target directory name for the sample.

I tried removing that functionality in the `gradle/gradle` code base, but I ran into long path issues on Windows: https://github.com/gradle/gradle/pull/5713.

So there should be a possibility to configure the target directory where a sample is copied to, either by specifying a fixed relative path to the target base dir or a target directory directly.

I think using a target directory supplied as a `File` would suffice. This field could then be wired in the method [computeSampleDir](https://github.com/gradle/exemplar/pull/6/files#diff-13f233a2042c31ac085fbc4aed7eeffaR123).

Review the Sample rule described in the issue and the computeSampleDir method referenced in the exemplar pull request. Trace how a sample's destination directory is selected and copied, then confirm the configured target directory is honored without breaking the existing default behavior.

Written by the indexing model from the issue text.