This diff adds a base64 crate to the Cargo Tunnelbroker dependencies.
base64 functions will be used in the following D5945 in a verify_signed_string function.

Linear task: ENG-2492

As this diff is a part of the stack this diff can be tested on the end of the stack at D5947.
To test it:

• Patch to the D5947
• Run the integration tests in D5931 using the cargo t --test tunnelbroker_integration_test command.

The expected result is passing the integration test (signing and verifying the string are successful and the new session creation test will pass).