Add configurable gzip compression level

[gdesrosiers1805]

The gzip layer previously used GZIPOutputStream with the JDK's default
compression level (level 6), with no way for schemas to choose a different
level. This change exposes the speed-vs-size trade-off that the gzip format
already supports, giving users control over how their data is compressed
during unparsing.

As a side benefit, this fix also addresses cross-zlib test failures
observed on Fedora 42. Fedora's OpenJDK links against zlib-ng while
Temurin and most other distributions bundle stock zlib. Both produce valid
gzip output, but the deflate token streams differ due to different
match-finding heuristics, causing tests with hardcoded byte baselines to
fail on Fedora. For the small test data used in Daffodil's gzip tests,
level 9 happens to produce identical output on both implementations,
letting the tests pass regardless of which zlib is linked. This
convergence at level 9 is empirical, not guaranteed.

Changes:

• Add compressionLevel DFDL variable to the gzip layer schema, defaulting
  to Deflater.DEFAULT_COMPRESSION (-1). Schemas can override via
  dfdl:newVariableInstance or dfdl:setVariable, and users can set the
  value externally without having to modify the schema.

• Add ConfigurableGZIPOutputStream, a GZIPOutputStream subclass that allows
  the compression level to be set via constructor argument.

• Update GZipLayer to accept the compressionLevel variable via
  setLayerVariableParameters and use it when constructing the output stream.

• Remove GZIPFixedOutputStream and the associated fixIsNeeded() method.
  These existed to work around a pre-Java-16 bug where GZIPOutputStream
  wrote 0x00 instead of the spec-compliant 0xFF for the gzip OS header
  byte. Since Daffodil's minimum supported Java version is now 17, this
  workaround is no longer needed.

• Add -parameters to javac options This preserves Java parameter names in
  bytecode, which is required by Daffodil's reflection-based layer parameter
  resolution. Without this, Java setters appear with parameters named arg0/arg1/...
  and cannot be matched to schema variables.

• Update test schemas (exampleGzipLayer.dfdl.xsd, exampleGzipLayer2.dfdl.xsd)
  to set compressionLevel=9 via newVariableInstance.

• Update TestGzipErrors.makeGZIPData to generate test input data at level 9
  for byte-stable output across JVMs, with comments documenting the
  empirical nature of the convergence.

DAFFODIL-3082

[stevedlawrence]

+1 👍 just a bunch of minor comments