ci: workflow to pre-build WASM binaries

[crepererum]

Closes #186.

[Copilot]

Pull request overview

This PR introduces automation for building and publishing pre-built WASM guest binaries through a GitHub Actions workflow. This simplifies integration for downstream users by eliminating the need for WASI compilation toolchains and reducing build times. The PR also refactors the disk space cleanup logic into a reusable composite action.

• Adds a new Prebuild workflow that builds WASM binaries for the Python guest and Rust example (debug and release variants), generates build attestations, and publishes them as GitHub releases
• Extracts disk space cleanup logic into a reusable composite action (.github/actions/free-disk-space)
• Updates documentation in CONTRIBUTING.md with instructions for triggering pre-built binary releases

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
.github/workflows/prebuild.yml	New workflow for building and releasing WASM binaries with attestation support
.github/actions/free-disk-space/action.yml	Composite action definition for freeing disk space on GitHub runners
.github/actions/free-disk-space/free.sh	Bash script that removes unnecessary files from GitHub runner images
.github/workflows/ci.yml	Updated to use the new reusable free-disk-space composite action
CONTRIBUTING.md	Added documentation section explaining pre-built WASM binaries and how to trigger builds

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[crepererum]

Here's a release created by this workflow (see docs in PR on how to trigger builds):

https://github.com/influxdata/datafusion-udf-wasm/releases/tag/wasm-binaries%2F2025-11-28T14-13-07%2F7aa299e7916e7e103281c152d6c75e20a6cf6a31