Add gcloud spanner databases compatible flags

[apstndb]

    gcloud spanner databases execute-sql (DATABASE : --instance=INSTANCE)
        --sql=SQL [--database-role=DATABASE_ROLE] [--enable-partitioned-dml]
        [--priority=PRIORITY] [--query-mode=QUERY_MODE; default="NORMAL"]
        [--timeout=TIMEOUT; default="10m"]
        [--read-timestamp=TIMESTAMP | --strong] [GCLOUD_WIDE_FLAG ...]

• --database-role as an alias of --role
• --sql as an alias of --execute Add --sql as alias of --execute #6
• --enable-partitioned-dml
• --query-mode Implement --query-mode and CLI_QUERY_MODE #44
• --timeout
• --read-timestamp and --strong Implements --string and --read-timestamp #48

    gcloud spanner databases ddl update (DATABASE : --instance=INSTANCE)
        [--async] [--ddl=DDL] [--ddl-file=DDL_FILE]
        [--proto-descriptors-file=PROTO_DESCRIPTORS_FILE]
        [GCLOUD_WIDE_FLAG ...]

• --async
• --ddl, --ddl-file
• --proto-descriptors-file Implement proto descriptor file functionality #18