[FEATURE]: Support customIndex for MySQL

[dengfuping]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Background

MySQL and MySQL ecosystem exists all kinds of create index scene‌s as below.

• create normal index:

CREATE INDEX idx_name on users(name) ;
CREATE INDEX idx_name on users(name) using hash;
CREATE UNIQUE INDEX idx_id on users(id);

• create spatial index:

CREATE SPATIAL INDEX idx_geom on users(geom);

• create fulltext index:

CREATE FULLTEXT INDEX idx_doc(doc) WITH PARSER NGRAM;

• create vector index for MySQL:

CREATE VECTOR INDEX idx_embedding ON users(embedding) SECONDARY_ENGINE_ATTRIBUTE=\'{"type":"spann", "distance":"cosine"}\';

• create vector index for OceanBase (a MySQL-like database but not 100% compatible):

CREATE VECTOR INDEX idx_embedding ON users(embedding) WITH (distance=L2, type=hnsw);

Proposal

Current Dizzle index couldn't implement these demands. So I proposal to add customIndex API to define raw create index statement.

const users = mysqlTable(
    "users",
    {
      id: bigint({ mode: "bigint" }).autoincrement().primaryKey(),
      name: varchar({ length: 255 }).notNull(),
      embedding: vector("embedding", { length: 3 }),
    },
    () => {
      return {
        idx_embedding: customIndex({
	  name: 'idx_embedding',
	  raw: 'CREATE VECTOR INDEX idx_embedding ON users(embedding) SECONDARY_ENGINE_ATTRIBUTE=\'{"type":"spann", "distance":"cosine"}\';',
	}),
        idx_fulltext: customIndex({
	  name: 'idx_fulltext',
	  raw: 'CREATE FULLTEXT INDEX idx_doc(doc) WITH PARSER NGRAM;',
	}),
      };
    }
  )

[dengfuping]

• Could cover vectorIndex requirement in [FEATURE]: Implement CREATE VECTOR INDEX for MySQL #3695