feature: Options to disable bignum support

[benluddy]

Is your feature request related to a problem? Please describe.

For an application whose CBOR-based protocol does not include arbitrary-precision integers (https://www.rfc-editor.org/rfc/rfc8949.html#section-5-3), it should be possible to reject the bignum tags 2 and 3 on decode and math/big.Int on encode.

Describe the solution you'd like

1. A new decode option specifying the decode behavior for the bignum tags, regardless of the Go type of the unmarshal destination object. Its default value would be backwards-compatible and would permit the bignum tag, but its second mode will cause Wellformed and Unmarshal to return an UnacceptableDataItemError if a bignum tag is encountered.
2. A new mode for the encode option BigIntConvert, analogous to the InfConvert and NaNConvert options introduced by Allow rejection of NaN and Inf float values on encode and decode. #513, that will return an UnsupportedTypeError if a math/big.Int is passed to Marshal.

Describe alternatives you've considered

The existing BigIntDec option is scoped to unmarshaling bignums into interface{}, which would not be suitable on its own for the purpose of rejecting all bignums. Bignums can also be decoded into uint, int, float, slice, array, and math/big.Int. I ended up adding the new option because I did not see a use case where the type of the destination Go value would affect whether or not a CBOR-based protocol accepts a particular input.

Additional context

[fxamacker]

Thanks Ben! 🎉 Closed by #527.