adding dav_chmod and dav_chown

[madara125]

Add chmod/chown support for WebDAV uploaded files

Description

This PR extends the nginx-dav-ext-module with three new directives that allow automatic permission management for files uploaded via WebDAV:

• dav_chmod: Set file permissions (supports octal notation, e.g., 0660)
• dav_chown_user: Set file owner (requires root privileges, included as a PoC)
• dav_chown_group: Set file group (requires root privileges, included as a PoC)

Problem Solved

Currently, nginx WebDAV module creates files with default system umask permissions, offering no control over ownership or access rights. This limitation makes it difficult to:

• Integrate WebDAV with automated workflows (e.g., Django/WordPress reading uploaded files)
• Implement collaborative editing workflows with privacy controls
• Apply security policies automatically to uploaded content

Implementation Details

• Added ngx_http_dav_ext_post_content_handler that executes in NGX_HTTP_LOG_PHASE after file creation
• Implemented octal permission parser for dav_chmod directive
• Added dav_chown_user and dav_chown_group directives with proper user/group resolution
• Minimal performance impact - only executes on successful PUT/MKCOL requests