feat: support anonymous table constraint style PRIMARY KEY

[apstndb]

Support anonymous PRIMARY KEY style as a table constraint inside the parentheses of CREATE TABLE.

This is the third way to define a primary key in Spanner, in addition to the inline column definition (PK INT64 PRIMARY KEY) and the trailing clause (PRIMARY KEY (PK)) outside the table elements. This style is supported by the Spanner emulator and actual database behavior.

Design decision

Introduced ast.TablePrimaryKey as a TableConstraint and updated Parser.parseCreateTable to support anonymous PRIMARY KEY inside the table elements while intentionally excluding named constraints.