Support custom mapping files in X12Reader

[AbhinavMir]

Adds the ability to parse X12 files against a user-supplied mapping definition, rather than being limited to the standards enumerated in X12Reader.FileType.

What changed

• New X12Mapping interface exposing the transaction definition and the expected ANSI version (the GS08 value). It includes a shared, secured XStream loader so built-in and custom mappings parse identically.
• FileType now implements X12Mapping.
• New CustomX12Mapping builds an X12Mapping from a user-supplied mapping XML stream plus a version string.
• X12Reader gains constructors accepting any X12Mapping (File / InputStream / Reader, with optional Charset). The existing FileType constructors delegate to them, so the public API is fully backward compatible.

Usage

try (InputStream mappingStream = new FileInputStream("/path/271.5010.X279.A1.xml")) {
    X12Mapping mapping = new CustomX12Mapping("005010X279A1", mappingStream);
    X12Reader reader = new X12Reader(mapping, new File("/path/file.txt"));
}

Tests

• testCustomMapping parses a 270 file through a custom mapping and asserts the result matches parsing the same file via the built-in FileType.
• testCustomMappingVersionMismatch verifies a wrong version is rejected.

README updated with a "Custom mapping files" section. All existing tests, checkstyle, and spotbugs pass.

[ctmay4]

@angelaszek can you please take a look?

[ctmay4]

Please fix merge conflicts.