feat(df): add gradle wrapper command behind the firewall proxy

Michael Eddingtonrequested to merge
df-mr-12-gradle-commands into main

Adds the Gradle package manager and the df gradle wrapper subcommand, completing the JVM ecosystem.

Gradle reuses the MavenMatcher (both resolve artifacts on the Maven repository path shape) and the JVM PKCS#12 truststore helper, setting GRADLE_OPTS to point at a generated store. The df gradle wrapper forwards all arguments verbatim to gradle.

Stacked on !3841 (merged) (the maven ecosystem MR), which itself carries the jvmtrust helper (!3840 (merged)) and the A→B→C command chain. Review/merge the stack in order.

Testing

  • go build ./...
  • GITLAB_CI=true go test ./internal/commands/df/... ./internal/dependencyfirewall/... — manager metadata, matcher type (Gradle uses MavenMatcher), no CA bundle var preserved.
  • gofmt and golangci-lint clean.
  • make gen-docs regenerated the gradle doc page.

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