base64.Encoding.Strict was added in #15656, but there was no equivalent method added to base32.Encoding.
To be consistent, I propose we add the equivalent functionality but rename it. The name Strict is unhelpful since there are multiple dimensions that the encoding could be stricter on.

I propose adding the following methods to both base32 and base64:

// RejectNonZeroPadding creates a new encoding identical to enc except
// that decoding requires trailing padding bits to be zero,
// as described in RFC 4648 section 3.5.
func (enc Encoding) RejectNonZeroPadding(enable bool) *Encoding 

and we deprecate the existing base64.Encoding.Strict method.