Expose underlying *spanner.Client from SpannerConn

[egonelbre]

Currently there are several issues that have come up that could be worked around with using *spanner.Client directly.

For example:

• Unable to scan JSON data into []byte or json.RawMessage #284
• Unable to scan into an array of struct #309
• Previously also structs not supported #281

By allowing to access *spanner.Client directly, it would make it easier to workaround issues that don't have yet parity in go-sql-spanner.

For workaround it's of course possible to use spanner.NewClient in addition to database/sql, but that would mean needing to manage multiple connection pools and also thread that access throughout the system.

It would be nice if you can use:

if err := conn.Raw(func(driverConn interface{}) error {
	spannerconn := driverConn.(spannerdriver.SpannerConn)
	client, err := spannerconn.UnderlyingClient()
	...
})

[egonelbre]

I'm not sure whether it's useful to make it:

UnderlyingClient() (any, error)
// instead of
UnderlyingClient() (*spanner.Client, error)

It seems unlikely that it will start using some completely new client. However, v2/v3 might necessitate to use a new client type.