Add support for foreign key to CREATE TABLE.

[matope]

Hi, I'm using your awesome library through handy-spanner.

Recently, Cloud Spanner supported the Foreign key feature and I'd like to use it on handy-spanner. This PR adds support FKs like below to CREATE TABLE in memefish. Could you take a look at this PR?

FOREIGN KEY (k1) REFERENCES T2 (t2key1)
CONSTRAINT foreignkey_name FOREIGN KEY (k1, k2) REFERENCES t2 (t2key1, t2key2)

ref:

• 外部キー  |  Cloud Spanner  |  Google Cloud
• 外部キー関係の作成と管理  |  Cloud Spanner  |  Google Cloud

[makenowjust]

Thank you!