Add Support for BigInteger / BigDecimal

[barbu110]

Can we use the rug crate? seems like a pretty mature crate on top of GNU libraries.

[rcoh]

I probably wouldn't pull in a crate that appears to be bound to a c library as my first choice, personally. But worth looking into.

This isn't especially high priority, I don't think there are an AWS models that use these shapes.

[timClicks]

To move forward with this, I recommend pulling in the num crate. It's pure Rust, well vetted and openly licensed.

[Velfi]

This would involve:

• Deciding whether or not we want to expose num types in our public API. It would be safer and more future-proof for us to newtype them instead.

• Updating SymbolVisitor.kt something like this:

  override fun bigIntegerShape(shape: BigIntegerShape?): Symbol {
      return symbolBuilder(RustType.Opaque(name = "BigInt", namespace = "num_bigint"))
          .addDependency(NumBigInt).build()
  }
  
  override fun bigDecimalShape(shape: BigDecimalShape?): Symbol {
      return symbolBuilder(RustType.Opaque(name = "BigRational", namespace = "num_rational"))
          .addDependency(NumRational).build()
  }

• Updating CargoDependency.ky like this:

   val NumBigInt: CargoDependency = CargoDependency("num-bigint", CratesIo("0.4.3"))
   val NumRational: CargoDependency = CargoDependency("num-rational", CratesIo("0.4.1"))

• Adding tests to ensure that we generate structs correctly when a smithy model specifies these new types