feat: add partial update support

[janlay]

Summary

Adds partial update support for WebDAV clients that require range-based writes, especially webdavfs.

This includes support for:

• PATCH with X-Update-Range
• PUT with Content-Range
• advertising sabredav-partialupdate via the DAV response header
• exposing Accept-Patch: application/x-sabredav-partialupdate

Motivation

Some WebDAV clients, such as miquels/webdavfs, probe for PUT range support before mounting read-write. Without partial update support, they fall back to read-only mode with:

no PUT Range support, mounting read-only

Advertising and implementing SabreDAV-style partial updates allows these clients to mount read-write.

Changes

• Intercept OPTIONS to advertise partial update capability.
• Handle partial updates before delegating to golang.org/x/net/webdav.
• Add PATCH permission handling equivalent to PUT.
• Add CORS defaults for PATCH, Content-Range, and X-Update-Range.
• Add tests for:
  • OPTIONS partial update headers
  • PATCH partial writes
  • sparse partial file creation
  • PUT with Content-Range

Testing

go test ./...
go build -o webdav .

Also manually verified with webdavfs on Debian:

sudo mount -t webdavfs -o"username=user,password=pass,allow_other,rw" http://10.0.0.2:6065 /mnt/test

The mount succeeds as read-write and no longer reports missing PUT range support. A mounted-file partial overwrite test also passed.