Add Base64 Encoding/Decoding and Enhanced String Handling for Bytes

[Shigoto-dev19]

Description

All changes revolve around the provable Bytes class.

Changes

• Added base64Encode and base64Decode methods.
• Added unit tests for the new Base64 methods.

Notes

• For detailed instructions on using Base64 encode/decode in o1js and an overview of provable Base64 constraints, refer to the o1js-base64 repo README.
• For an example of zkPrograms using Base64, see this file.

[mitschabaude]

note that the tests are just failing because you changed the bindings and mina submodules

[Shigoto-dev19]

I reviewed it, have a few more comments, looks great overall

Thank you for the review! I'll address the comments and update the branch :)