In order to make use of libproxy in a flatpak environment, we need to
support XDP proxy resolver. Add support.

Currently CI fails due to gi-docgen error:

 FAILED: docs/libproxy-1.0
/opt/homebrew/bin/gi-docgen generate --quiet --add-include-path=/Users/runner/work/libproxy/libproxy/docs/src --config=docs/px.toml --output-dir=docs/libproxy-1.0 --no-namespace-dir --content-dir=/Users/runner/work/libproxy/libproxy/docs src/libproxy/Libproxy-1.0.gir
/opt/homebrew/Cellar/gi-docgen/2023.3/libexec/lib/python3.12/site-packages/typogrify/filters.py:46: SyntaxWarning: invalid escape sequence '\s'
  tag = match.group(1) if match.group(1) != '' else '[^\s.#<>]+'
/opt/homebrew/Cellar/gi-docgen/2023.3/libexec/lib/python3.12/site-packages/typogrify/filters.py:140: SyntaxWarning: invalid escape sequence '\w'
  tag_pattern = '</?\w+((\s+\w+(\s*=\s*(?:".*?"|\'.*?\'|[^\'">\s]+))?)+\s*|\s*)/?>'
/opt/homebrew/Cellar/gi-docgen/2023.3/libexec/lib/python3.12/site-packages/typogrify/packages/titlecase/__init__.py:18: SyntaxWarning: invalid escape sequence '\.'
  SMALL = 'a|an|and|as|at|but|by|en|for|if|in|of|on|or|the|to|v\.?|via|vs\.?'
ERROR: Could not find GIR dependency in the search paths: Gio-2.0
ninja: build stopped: subcommand failed.