Add support for 'OnUpdate'

[ting-lan-wang]

Description

Fixes TestForeignKeyConstraints and TestForeignKeyConstraintsBelongsTo.

1. Change the tag from uniqueIndex to unique

Oracle does not treat a unique index the same as a unique constraint.
When a column is tagged with uniqueIndex, GORM generates a unique index, but foreign key constraints in Oracle require the referenced column to be backed by a unique constraint.

For example, given the following structs:

	type Profile struct {
		ID       uint
		Name     string
		MemberID uint
	}

	type Member struct {
		ID      uint
		Refer   uint `gorm:"uniqueIndex"`
		Name    string
		Profile Profile `gorm:"Constraint:OnUpdate:CASCADE,OnDelete:CASCADE;FOREIGNKEY:MemberID;References:Refer"`
	}

the generated SQL is:

CREATE TABLE "members" ("id" NUMBER(20) GENERATED BY DEFAULT AS IDENTITY,"refer" NUMBER(20),"name" VARCHAR2(4000),PRIMARY KEY ("id"))

CREATE TABLE "profiles" ("id" NUMBER(20) GENERATED BY DEFAULT AS IDENTITY,"name" VARCHAR2(4000),"member_id" NUMBER(20),PRIMARY KEY ("id"),CONSTRAINT "fk_members_profile" FOREIGN KEY ("member_id") REFERENCES "members"("refer") ON DELETE CASCADE)

Which fails with:
ORA-02270: no matching unique or primary key for this column-list

Changing the tag to unique generates the correct SQL:

CREATE TABLE "members" ("id" NUMBER(20) GENERATED BY DEFAULT AS IDENTITY,"refer" NUMBER(20),"name" VARCHAR2(4000),PRIMARY KEY ("id"),CONSTRAINT "uni_members_refer" UNIQUE ("refer"));

CREATE TABLE "profiles" ("id" NUMBER(20) GENERATED BY DEFAULT AS IDENTITY,"name" VARCHAR2(4000),"member_id" NUMBER(20),PRIMARY KEY ("id"),CONSTRAINT "fk_members_profile" FOREIGN KEY ("member_id") REFERENCES "members"("refer") ON DELETE CASCADE)

2. Support OnUpdate

Unlike other databases, Oracle does not directly support the ON UPDATE foreign key constraint. To work around this, the driver generates a trigger that simulates the ON UPDATE behavior. When a constraint is tagged with OnUpdate, the driver skips generating the unsupported SQL and instead creates a trigger that cascades updates whenever the referenced column in the parent table changes.

The trigger is automatically dropped when the table is dropped or when DropConstraint() is called.

Added a test TestMigrateOnUpdateConstraint to verify this feature.