RFR: 8338405: JFR: Use FILE type for dcmds

[Sonia Zaldana Calles]

Hi all, 

This PR addresses [8338405](https://bugs.openjdk.org/browse/JDK-8338405) switching `STRING` to `FILE` for JFR.start, JFR.stop and JFR.dump where appropriate. 

This is consistent with the new `FILE` type that was introduced in [8334492](https://bugs.openjdk.org/browse/JDK-8334492).

Testing:
- [x] JFR test suite passes 
- [x] GHA 

Thanks, 
Sonia

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Commit messages:
 - 8338405: JFR: Use FILE type for dcmds

Changes: https://git.openjdk.org/jdk/pull/21025/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21025&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338405
  Stats: 96 lines in 5 files changed: 48 ins; 38 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/21025.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21025/head:pull/21025

PR: https://git.openjdk.org/jdk/pull/21025