Default Value: Spanner Metadata Accessor

[NirnayaSindhuSuthari]

• DefaultValue of type DefaultValue is added in the columnDef struct.
• Validity of Default Value by spanner is checked by IsSpannerSupportedStatement .
• isValidSpannerStatement fires query SELECT CAST( (statement) AS (column type) ) to spanner on metadata database.
• Default clause is added in the SQL tab for spanner supported default values.