add metadata feature

[spechtx]

Add PDF Metadata Support

This PR adds support for PDF metadata manipulation via Gotenberg's metadata API.

Features

• Adds MetadataMixin for setting PDF metadata fields including:
  • Document information (title, author, subject, keywords)
  • Dates (creation, modification)
  • Technical details (pdf version, creator, producer)
  • PDF standards (trapped status, marked status)
• Full validation of metadata values
• Support for method chaining
• Comprehensive test coverage

Example Usage

route.metadata(
    title="My Document",
    author="John Doe",
    creation_date=datetime.now(),
    keywords=["sample", "document"],
    subject="Sample PDF Generation"
).run()

Documentation

Added metadata examples to README.md
Added documentation for all metadata fields and options
Included PDF/A and metadata section in the documentation

Notes

The existing test failures are unrelated to this implementation (verified against main branch)
All metadata fields are optional
Values are properly validated before being sent to Gotenberg

[stumpylog]

I haven't looked at all the changes yet, but besides the linting so tests can run, I would like a few other changes:

• It's great to see testing, but
  • Let's group tests into classes so it follows the same format as existing ones
  • Let's use some more end to end testing that actually goes through the client, and the result is read via pikepdf or similar to ensure it is applied
• I think the "Trapped" should be more strongly typed than a string or bool, you've created the enum, so it seems good to use it