Refactor Core API to Fluent Model & Functionality Extension

[vanbroup]

This Pull Request represents a comprehensive overhaul of the pdfsign library. It introduces a major API refactoring to a modern Fluent API model and significantly extends functionality across the board.

We have moved away from the legacy functional options pattern to improve developer experience and added robust support for new capabilities like advanced appearance customization, form handling, and comprehensive external signer integrations.

Key Changes

1. Fluent API Model

We have replaced the legacy SignWithOptions and VerifyWithOptions patterns with a chainable builder interface.

Before (Legacy):

options := pdfsign.SignOptions{
    Signature: pdfsign.SignData{
        SignatureType: pdfsign.DocTimeStamp,
    },
    Signer: signer,
}
err := pdfsign.SignFile("input.pdf", "output.pdf", options)

After (New Fluent API):

err := pdfsign.OpenFile("input.pdf").
    Sign().
    SetType(pdfsign.DocTimeStamp).
    SetSigner(signer).
    WriteToFile("output.pdf")

2. Extended Functionality

We have significantly expanded the library's capabilities:

• Advanced Appearance: Added support for custom fonts, vector graphics (shapes, lines), PDF objects (embedding pages), and initials.
• Forms: Implemented comprehensive support for reading and filling PDF form fields.
• Verification Logic: Stricter and more robust PDF signature verification.

3. External Signer Integrations

Added distinct modules providing implementations for major remote signing services. These serve as production-ready examples of how to integrate external KMS providers without bloating the core library.

• AWS KMS: signers/aws
• Google Cloud KMS: signers/gcp
• Azure Key Vault: signers/azure
• CSC (Cloud Signature Consortium): signers/csc
• PKCS#11: signers/pkcs11

4. Code Quality & Testing

• Unit Tests: Comprehensive *_test.go coverage added for all packages.
• Fixtures: Standardized test PDFs and assets in testfiles/.
• Linting: Resolved errcheck, govet, and staticcheck issues; strict linting enforced.

Breaking Changes

• The pdfsign.Sign() and pdfsign.Verify() top-level functions are deprecated and will be removed in a future release. Please migrate to pdfsign.Open().Sign() and pdfsign.Open().Verify() immediately.
• Top-level types have been reorganized; check types.go for new definitions.

Verification

• Run go test ./... to execute the new test suite.
• Run scripts/verify_pdfs.sh to validate generated signatures against DSS (if configured).