[keyserver] Removing .gitignore from rust-node-addon

Summary:
This diff removes .gitignore file from the rust-node-addon and adds ignoring directories to the global .gitignore file at the root of the project. It makes sense to use a root gitignore file to ignore these two directories.

Linear task: ENG-3048

Test Plan: Running yarn clieaninstall will build the rust-node-addon as a dependency. The building process produces files in the target and napi subdirectories which are ignored.

Reviewers: varun, jon, tomek, ashoat

Reviewed By: varun, jon, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6792