ADD COLUMN IF NOT EXISTS support in emulator

[HengzhiShao-ANZ]

The following works on Google Cloud Spanner console but not in spanner emulator:

ALTER table TableName
ADD COLUMN IF NOT EXISTS ColumnName BOOL NOT NULL DEFAULT (true);

Getting Syntax error on line _, column _: Encountered 'IF' while parsing: identifier failed to init spanner emulator

[mohitg1580]

This has been fixed as part of this - https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/issues/119

[HengzhiShao-ANZ]

This has been fixed as part of this - #119

Great! I guess this issue can be closed now.