Add validator options to change priority of snapshot packager and RPC threads (v1.8) (#21020)

* Add function for changing thread's nice value

Linux only.

* Add validator option to change niceness of snapshot packager thread

* Add validator option to change niceness of RPC server threads

Fixes https://github.com/solana-labs/solana/issues/14556

* Run `./scripts/cargo-for-all-lock-files.sh tree`

core/src/validator.rs

@@ -681,6 +681,7 @@ impl Validator {
                     &exit,
                     &cluster_info,
                     snapshot_config.maximum_snapshots_to_retain,
+                    snapshot_config.packager_thread_niceness_adj,
                 );
                 (
                     Some(snapshot_packager_service),