Add `--set` flag to `cargo msrv (find)`

[foresterre]

This flag should:

• Set or update the MSRV when found, and the crate has a [package] table (ie is not a virtual workspace)
• Retain the TOML styling if already exists
• Add MSRV as rust-version key/value pair in the [package] table
  • Or: add as msrv key/value pair in the [package.metadata] table when before Rust 1.56 (<=1.55)

Alternative name ideas:

--update, --apply --apply-msrv