Add proxy support

[glichtner]

Add Proxy Support to git2r

This pull request introduces proxy support to the git2r package.

Summary of Changes

• Proxy Argument Addition:
  R Functions clone, fetch, push, and remote_ls now include a proxy parameter, enabling users to define proxy settings directly within these functions.

• Proxy Options Handling:
  Implementation of internal functions to manage proxy settings, including validation and integration with libgit2's proxy options.

• Documentation Updates:
  Enhancements to the documentation to reflect the new proxy parameter, providing guidance on its usage and possible values.

• Test Coverage:
  Added basic tests for all functions that implement the proxy parameter.

Details

Functions Updated

• clone: Added proxy parameter to specify proxy settings during repository cloning.
• fetch: Included proxy parameter to define proxy settings when fetching updates from a remote repository.
• push: Integrated proxy parameter to set proxy settings during push operations.
• remote_ls: Enhanced to accept a proxy parameter for listing references in a remote repository through a proxy.

Proxy Parameter

The proxy parameter takes the following values (corresponding to libgit2`s git_proxy_t):

• NULL: No proxy used (default)
• TRUE: Try to auto-detect the proxy from the git configuration.
• character vector: Connect via the URL given in the options

Internal Functions

• git2r_set_proxy_options: New function to initialize and set proxy options based on user input.
• git2r_arg_check_proxy: Validation function to ensure the provided proxy arguments are of acceptable types (NULL, TRUE, or a valid string).

Documentation

• Updated manual pages for all modified functions to include descriptions and usage examples for the proxy parameter.

Related issues

Should fix #395 , #278, and possibly #269 .

[stewid]

Thanks for the pull request. I'll check it out in the next few days.

[stewid]

Thanks

[glichtner]

@stewid Happy to contribute!

I'm not sure why the CI tests failed on ubuntu-latest (devel) after merging (https://github.com/ropensci/git2r/actions/runs/12797680530/job/35680105737). They succeeded in the PR. Is it possible that some of the ubuntu runners on github do not have libgit2 installed, and that the CI workflow should run a "install libgit2" not only on macos runners, but on all runners, just for good measure?

Possible change to the workflow file could be:

- name: Install libgit2 on macOS and Ubuntu
  run: |
    if [ "$RUNNER_OS" == "macOS" ]; then
      brew install libgit2
    elif [ "$RUNNER_OS" == "Linux" ]; then
      sudo apt-get update && sudo apt-get install -y libgit2-dev
    fi
  shell: bash