Web and pdflib enhancements

[dcbr]

Awesome library! I would like to use it for web applications, in combination with the pdf-lib library, for which following enhancements are required or useful.

Overview

• [Web]: As there is no node:Buffer type, web applications are not supported right now (type error). This PR accepts beside the Buffer also the Uint8Array (both available in Node and Web applications) and string (base64 encoded bytes) types when signing a pdf or creating a P12Signer. An extra utility function convertBuffer is introduced for this.
• [Pdflib]: The pdflib placeholder now accepts a PDFDocument or PDFPage input, supporting signatures on other pages than the first one.
• [Pdflib]: The pdflib placeholder now accepts a signingTime input, such that it can be matched with the document creation or modification time.
• [Pdflib]: The pdflib placeholder inserts the signature dictionary as a PDFInvalidObject, such that it is not serialized in an object stream. This means the {useObjectStreams: false} is no longer needed when saving the document (and thus compressing the final signed pdf size).
• [Pdflib]: The pdflib placeholder inserts an appearance stream dictionary for the signature widget, which is required for PDF/A compliance.
• [Pdflib]: The pdflib placeholder inserts a Prop_Build dictionary for the signature. Allowing to specify which filter and application were used to generate the signature.
• [Sign]: The sign function and Signers have an extra signingTime input, such that it can be matched with the document creation or modification time.

[vbuch]

Reading through your description, this seems like an exciting contribution. I'll give it a more thorough look in the coming days but I love every word of the description.

[vbuch]

I'll get that released today or tomorrow. Thank you for a great contribution!