feat: Add Aviator source encoding handling

[kireetivar]

Summary

• Add configurable Aviator source file encoding handling through --source-encodings, defaulting to FPR,UTF-8,CP850.
• Use FPR source encoding metadata when decoding source files, with fallback encoding candidates for audit and remediation flows.
• Propagate source encoding options through Aviator SSC audit, SSC/FoD apply-remediations, FPR processing, source enrichment, and gRPC calls.
• Improve Aviator audit results by reporting skipped issue counts, skipped reasons, and remediation-generation skip metrics.

feat: `fcli aviator ssc audit`: Add `--source-encodings` option for source decoding and audit skip reporting
feat: `fcli aviator ssc apply-remediations`: Add `--source-encodings` option for source file decoding and encoding
feat: `fcli fod aviator apply-remediations`: Add `--source-encodings` option for source file decoding and encoding

[rsenden]

I haven't reviewed everything in detail, focusing on overall structure first. Instead of having custom option parsing functionality, it would be much better to use existing picocli functionality; something like the following:

• Create ISourceDecoder interface, with single decode method matching signature of current SourceEncodingOptions::decode
• Create implementations of this interface:
  • One implementation in which the decode method retrieves encoding from FVDL
  • One that takes a fixed Charset as constructor arg (together with a String-based constructor that calls Charset.forName)
  • Optionally a CompositeSourceDecoder that takes a List<ISourceDecoder> as constructor argument
• Create a corresponding picocli ITypeConverter that based on input (FPR or explicit charset) returns an instance of one of the classes above, throwing a proper picocli exception like TypeConversionException to have picocli render a proper error message in case of invalid charset
• Create picocli mixin/arggroup class (ideally in fcli-aviator-common for re-use across FoD/SSC, but not sure whether we currently have any picocli-related classes in this module) that defines --source-encodings as List<ISourceDecoder> (or directly as CompositeSourceDecoder, but not sure whether that would work with split), with proper split, converter, and defaultValue attributes
• Pass this list of candidate decoders to the processors (either as-is, or wrapped in CompositeSourceDecoder, but for flexibility, processors would rely on ISourceDecoder interface)
• For encoding, have a separate SourceEncoder class with static encode method, as this operates on specified Charset and doesn't (directly) rely on any of the above