feat: replace log with tracing and use tracing::instrument where useful

[simonsan]

Fixes #315

TODO:

• [x] replace log with tracing
• [x] annotate functions/methods with tracing::instrument, candidates:
  • async, e.g. WebDavFS and opendal backend
  • in general multithreading (TreeArchiver)
  • contain *_par_*/readahead* via rayon
  • send/recv data on channels
• [ ] add SpanTrace to RusticError for https://crates.io/crates/tracing-error
• [ ] add more fine-grained spans and logging for above impl, e.g. in closures where we spawn threads
• [ ] add debug and trace at some more sensible places to make diagnosing backend issues more easy (ref.: https://github.com/rustic-rs/rustic_core/pull/346 )
• [ ] what else to prepare, that we can use the instrumented library in rustic-rs?
  • [ ] See https://github.com/rustic-rs/rustic/pull/1342