Set MSRV for clippy in rustworkx (Qiskit#793)

Starting in the recently released Rust 1.67.0 clippy enabled a new rule
by default, uninlined_format_args [1], which warns when format!() is
used and a variable is not put in the string inline but isntead as a
separate argument in the macro. For example, `format!("{}", foo)`
Instead the rule suggests using the variable inline the format call
(e.g. `format!("{foo}")`. However, with our msrv of 1.56.1
this syntax was not supported. So trying to fix the places the rule is
triggered as suggested will fail to compile with rust 1.56.1. To address
this and solve this for future rust releases and potential new rules this
commit adds a clippy.toml file which sets the msrv for the rustworkx crate
This will by default disable rules that weren't on by default in rust
1.56.1.

[1] https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args

clippy.toml

@@ -0,0 +1 @@
+msrv = "1.56.1"