build: bundle ggshield binary with extension

[6d7a]

Context

Currently, the extension downloads ggshield at runtime. This can cause issues in environments with no or restricted network access.

What has been done

This PR replaces the runtime download with a build-time one. The ggshield binary is downloaded and bundled with the extension in a platform-specific extension. The PR also includes a configuration parameter gitguardian.ggshieldPath that allows to override the default of the bundled ggshield.

Didn't we do this before?

We used to bundle ggshield in this extension until e99b365. This approach differs as follows:

• binaries are not kept in the repo tree but downloaded at build-time from ggshield GitHub releases
• we ship platform-specific extensions instead of a single extension containing binaries for all platforms

Validation

1. Build and install the VSIX: vsce package --target darwin-arm64 --allow-missing-repository && code --install-extension gitguardian-*.vsix
2. Reload VS Code — verify the GitGuardian sidebar loads and scanning works

PR check list

• As much as possible, the changes include tests
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry.