feat: support custom logger

[eduardoboucas]

Summary

This PR adds a logger property to buildSite. When present, logs will continue to be streamed but printed using this custom function instead of console.log, letting consumers customise how logs will be processed (e.g. if logger is set to console.error, logs will be piped to stderr instead).

The buffer property, which tells Netlify Build to buffer logs and return them as a set of stderr/stdout arrays, takes precedence.

[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.