Add a `--log-level=<level>` CLI flag to `forc` as an alternative to using `RUST_LOG` env var

[mitchmindtree]

Currently, the only way to filter certain kinds of logging output is to specify a filter level via RUST_LOG, e.g. RUST_LOG=trace, RUST_LOG=off, and so on.

It could be useful to provide a CLI flag alternative for this, e.g. --log-level "warn". This would have the added benefit of appearing in our auto-generated forc documentation, and is arguably a bit cleaner than requiring uses pass a RUST_LOG environment variable.

Any --verbose options could act as an alias for --log-level "debug" or --log-level "trace".

We would need to decide on what option should take precedence in the case that both RUST_LOG and --log-level were specified. Perhaps we should select the highest-level of the two.

Originally raised here https://github.com/FuelLabs/sway/pull/2308#pullrequestreview-1036769377.