RFR: [Fibula] Make OutputFormatAdapter public

[Galder Zamarreño]

This is the first in a series of PRs that I will be sending over the next few days, which make extending JMH to benchmark Java code running as GraalVM native images more easily. The codename for this extension is Fibula. It can still work without this PRs but without them the extension has to duplicate code that exists in JMH, which is not desirable.

In this PR, we make `OutputFormatAdapter` public.

Why do that? This enables Fibula to reuse the same class when it wants to redirect the output from the process to the JMH output. This is useful for example when Fibula's PGO integration invokes native image to recreate the binary with the profiling information, see https://github.com/galderz/fibula/commit/20eaf64936ef3e8f77eaed2ac7e8094b57e66a8d

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Commit messages:
 - Make OutputFormatAdapter public

Changes: https://git.openjdk.org/jmh/pull/158/files
  Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=158&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmh/pull/158.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/158/head:pull/158

PR: https://git.openjdk.org/jmh/pull/158