feat: estimate transaction size in the FFI crate

[CiottiGiorgio]

I thought this code is common enough we might want it in core.

I am still missing a way to do it in the rust crate. After I do that, the FFI crate should only re-expose what's in the core crate.

Also, I haven't taken the approach of actually signing it as it a slow operation. I decided to attach an invalid signature, encode, and get the size.

I've also seen a bunch of times just adding 75 to the encoding length without a signature. I am assuming 75 comes up from the field name, the encoding and the signature length.
What approach do we think is best?