Add support for unsafe extern blocks

[pvdrz]

This PR adds support for the unsafe_extern_blocks feature which was stabilized for Rust 1.82.

This feature is activated whenever the Rust target is greater or equal than 1.82.

Fixes #2901

cc pgcentralfoundation/pgrx#1810

[emilio]

I didn't review all the test changes but given the patch I'm pretty confident they're just more unsafe keywords. So looks good :)

[pvdrz]

we're getting lots of semantic merge conflicts lately 🤣

[Kriskras99]

I think there might be a bug with the version matching:

error: extern block cannot be declared unsafe
    --> /home/christiaan/scate2-dev/rust/target/release/build/flexnet_client-sys-a77799b3d52b9432/out/bindings.rs:3363:1
     |
3363 | unsafe extern "C" {
     | ^^^^^^
     |
     = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information

error: extern block cannot be declared unsafe
    --> /home/christiaan/scate2-dev/rust/target/release/build/flexnet_client-sys-a77799b3d52b9432/out/bindings.rs:3371:1
     |
3371 | unsafe extern "C" {
     | ^^^^^^
     |
     = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information

error: could not compile `flexnet_client-sys` (lib) due to 243 previous errors
warning: build failed, waiting for other jobs to finish...
christiaan@CBHST34:~/scate2-dev/rust$ rustc --version
rustc 1.81.0 (eeb90cda1 2024-09-04)

This is with bindgen = { version = "0.71.1" }