Add CA certificate bundle path override for SSL issues

[bananna-droid]

Description

Adds support for custom CA certificate bundle path configuration to resolve SSL certificate validation failures.

Fixes

#146

Affected Platforms

This issue affects all platforms (Linux, Windows, macOS), not just Linux:

• Linux: Confirmed on Ubuntu 24.04 / Pop!_OS 24.04 (tested by me)
• Windows: Confirmed on Windows 10 (tested by me)
• macOS: Likely affected (same root cause)

The plugin appears to be built with OpenSSL on all platforms, which fails to locate system CA certificates without proper configuration.

Testing

• ✅ Tested and working on Linux (Ubuntu 24.04, Pop!_OS 24.04) with Godot 4.5.1 and 4.6
• ✅ Tested and working on Windows 10 VM with Godot 4.6

Changes

• Added ca_bundle_path member and set_ca_bundle_path() method to GitPlugin
• Implemented automatic config loading from ca_cert_config.txt
• Uses git_libgit2_opts(GIT_OPT_SET_SSL_CERT_LOCATIONS) for runtime certificate configuration
• Included template config file with examples for common Linux distributions

Testing

Tested on Linux with custom CA bundle path. SSL operations (fetch/pull/push) now work correctly when config file is present.

Behavior

• Opt-in: No changes to default behavior
• Zero impact: Users without the config file experience no changes
• User-friendly: Simple text file configuration, no code modifications needed

Example Config File

# ca_cert_config.txt
/etc/ssl/certs/ca-certificates.crt

Rationale

While ideally libgit2 would auto-detect certificate locations on all systems, this provides users immediate control in environments where auto-detection fails. This follows the same pattern as curl's --cacert and git's http.sslCAInfo options.