feat: Make attach pub

[MichaReiser]

It's sometimes necessary to print salsa data outside a query. However, the output isn't very useful without a db attached
and salsa currently doesn't expose an API to attach the db.

This PR makes attach::attach pub, so that user-code can attach a database when necessary.

My specific use case is that I want to print a salsa::Backtrace that was captured earlier but the handler
isn't part of any salsa query.

Alternatives: We could add a Backtrace::display method that accepts a specific db.

Downsides: This exposes that each db is associated with a single thread. I think this
is something that's already observable to users anyway.

[netlify]

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[codspeed-hq]

CodSpeed Performance Report

Merging #832 will not alter performance

_{Comparing MichaReiser:make-attach-pub (37d5e5e) with master (af811a3)}

Summary

✅ 12 untouched benchmarks