fix(spanner): MarshalJSON function caused errors for certain values

[danysousa]

This pull request addresses an issue in the implementation of the Marshaler interface within the spanner package.
The current implementation leads to errors when dealing with certain special characters, such as certain emojis. To resolve this issue, I have adopted an approach similar to that used in the bigquery package, which is not affected by this problem.

The root cause of the problem lies in the use of Sprintf, which may seem appealing but lacks the advantages of checks and escapes provided by the encoding/json package. This pull request aims to rectify the situation by using the json.Marshal.

To validate the proposed solution, a small sample code is provided below that reproduces the error with the current implementation and demonstrates that the proposed changes resolve the issue:

package main

import (
    "encoding/json"
    "cloud.google.com/go/spanner"
)

func main() {
    value := spanner.NullString{
        Valid:     true,
        StringVal: `🩷`,
    }

    marshalledValue, err := json.Marshal(value)
    if err != nil {
        println("Error:", err.Error())
    } else {
        println("Value:", string(marshalledValue))
    }
}

Output before the fix :

Error: json: error calling MarshalJSON for type spanner.NullString: invalid character 'U' in string escape code

Expected output :

Value: 🩷

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