feat(spanner): implement valuer and scanner interfaces

[olavloite]

Valuer / Scanner Interfaces

Adds implementations for the driver.Valuer and sql.Scanner interfaces for the Spanner Null* types. This makes it possible to use both the spanner.Null* types and the underlying native types in the Go sql driver.

That is, both the following will then be supported:

var r spanner.NullNumeric
rows.Scan(&r)

AND

var r big.Rat
rows.Scan(&r)

It is not possible to implement this directly in the Go sql driver, as these types are defined in the spanner package.

The interfaces are not implemented for the spanner.NullJSON type for two reasons:

1. NullJSON already has a field called Value, which makes it technically impossible to add a method called Value().
2. The underlying value of NullJSON is of type interface{}, which means that it can be anything. This means that there is no relevant other type than NullJSON that a user can use when calling sql.Row#Scan(dest ...interface{}) for a JSON column.

Gorm Data Type

Adds default data type mappings for the spanner.Null* types. That is; NullInt64 is for example mapped by default to an INT64 column. This allows structs that use spanner.Null* types for its fields to be used directly in Gorm migrations without the need to annotate them with the data type they should have in the database.
This feature is implemented by adding the func GormDataType() string to each of the spanner.Null* types.

[hengfengli]

How about JSON? Why does it not implement Value and Scan interfaces?

[olavloite]

I ran into a problem with JSON, and that is that spanner.NullJson has an exported field called Value. Go does not allow a struct to have a method and a field with the same name, so we cannot add the Value() method to NullJson. At least not without making a breaking change by renaming the Value field to something else.

[hengfengli]

Can we add this problem to the PR description? or the comment in the code? Just for records that why we don't implement valuer and scanner interfaces for JSON.

[olavloite]

Good point, I've added it both to the comments for NullJSON and to the PR description.

[hengfengli]

LGTM.