[native/rust_library] Adding sign_string_with_private_key function to crypto tools

Summary:
This diff introduces adding the sign_string_with_private_key function to crypto tools. This function gets the string to be signed and the private key and returns a sha256 signature. We will use this function in the Tunnelbroker client's new session creation because we need to provide the signed string to the Tunnelbroker to prove the ownership of the keys.

Linear task: ENG-1333

Test Plan: Successfully passing the unit test for this function in D6000

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5999