feature: Add SCALE custom type functionality for Result, VaryingDataType, and VaryingDataTypeSlice

[timwu20]

Task summary

• SCALE requires the destination data structure to be known before decoding. This isn't a problem when we deal with initialised Result, VaryingDataType and VaryingDataTypeSlice instances.
• The issue is when Result, VaryingDataType and VaryingDataTypeSlice are used as attributes in structs. The zero case for the Result, VaryingDataType and VaryingDataTypeSlice have not been intialized with their associated value types, which causes a decode error.
• The proposed solution is to allow for shadowing (custom types) of Result, VaryingDataType and VaryingDataTypeSlice, and to utilise the constructor pattern (ex. func NewMyResult() *NewResult) which will be called by the scale decoding process to initialise these special types. In the event that there is no constructor defined in the package we could throw a friendly error to notify the developer.

Specification reference

Other information and links

• refactor(lib/scale): refactor lib/scale into own package with idiomatic types, marshalling, and optionality #1548

[danforbes]

Is this a place where we could be able to make use of generics?