Umbrella issue of minor breaking changes

[apstndb]

This issue is memo of minor breaking changes(non-mandatory, but better to do).

• TABLESAMPLE clause may be better to move to FROM and JOIN clause from all TableExpr implementations
• AsAlias([AS] name) may be better to be renamed as Alias, and call AS name as AsAlias.
  • Discussed in GCPUG Slack
  • Related discussion: we have not yet concluded, but I think there is a benefit to name expr AS name(NamedExpr?), and expr [AS name] (ZetaSQL's ASTExpressionWithOptAlias)
• SimpleType can be removed because NamedType can handle all SimpleType.