[Lib] Implement base64url functions

[anakrish]

Implement base64url functions and add tests.

• base64url.decode
• base64url.encode
• base64url.encode_no_pad

Ensure that cargo test --test opa -- base64url passes.

See

• https://github.com/microsoft/regorus/blob/main/src/builtins/conversions.rs for how to implement library functions.

• regorus/src/builtins/mod.rs

  Line 49 in a75f7c5

  conversions::register(&mut m);

  shows how to register the functions.

• regorus/tests/interpreter/cases/builtins/types/tests.yaml

  Line 4 in a75f7c5

  cases:

  shows how to add tests.

[anakrish]

Implemented by #66