Use Zowe encoding for DSN data

[Tannex]

Summary

• resolve the Zowe encoding property through base, nested profile, team/user layer, and ZOWE_OPT_ENCODING precedence
• use that value for --data-dsn decoding when -codepage is not explicitly supplied
• preserve explicit -codepage, the cp037 fallback, and lazy Zowe loading for local inputs
• document the behavior and add focused parser and end-to-end coverage

Branch relationship

This branch is synchronized with main at merge commit 33ec3ab, which contains PR #9. Merge commit 31281b7 resolves the former stacked-branch conflicts, so this PR now shows only the encoding-specific changes.

Validation

Rerun on head 31281b7:

• go test ./...
• go test -race ./...
• go vet ./...
• Windows amd64 test-binary cross-compile
• macOS arm64 test-binary cross-compile