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Describe the enhancement you want to request

Context / prior discussion: #2625

Problem

Drizzle’s Postgres bigint() currently supports mode: 'number' | 'bigint', but not a string mode. Many ecosystems (Discord snowflakes, some external APIs, JSON pipelines) treat 64-bit IDs as strings to avoid precision issues and BigInt JSON pitfalls. Today the only workaround is a customType that converts to/from BigInt, but that breaks/limits ergonomics: you can’t use .generated…AsIdentity() on a customType, and relations are fragile/missing with custom types.

My minimal workaround (works, but limited)

export const bigint = customType<{
  data: string;
  driverData: string | number | bigint;
}>({
  dataType: () => 'bigint',
  toDriver: (v) => {
    if (typeof v !== 'string' || !/^-?\d+$/.test(v)) {
      throw new Error(`Invalid bigint string: ${v}`);
    }
    return BigInt(v);
  },
  fromDriver: (v) => (typeof v === 'string' ? v : v.toString()),
});

Limitations: cannot combine with .generatedAlwaysAsIdentity() / .generatedByDefaultAsIdentity() and doesn’t integrate cleanly with relations.

Requested feature
Add a third mode for Postgres bigint/bigserial/identity columns:

id: bigint({ mode: 'string' }).primaryKey().generatedAlwaysAsIdentity();

What “string mode” should do

• DB type: still BIGINT in Postgres.
• TS types: select -> string, insert -> string | undefined (depending on identity).
• Driver mapping: automatic string ⇆ BigInt conversion similar to the customType above (internally), so users don’t touch BigInt at all.
• Works with: .generatedAlwaysAsIdentity() and (if/when supported) .generatedByDefaultAsIdentity(); sequence options remain available.
• Relations: first-class support in relations/foreign keys (no “customType” edge cases).

Why this is valuable

• Avoids precision issues and BigInt JSON serialization errors in common stacks.
• Matches how many APIs represent 64-bit IDs (strings).
• Keeps the great DX of identity columns and relations—without customType foot-guns

Backward compatibility

• Non-breaking: add string alongside existing number | bigint ; current behavior stays default.
• Docs: a short note in PostgreSQL -> bigint explaining when to pick string (JSON/web APIs), vs number (≤ 2^53), vs bigint (native bigint).

Thanks! This would remove a common friction point when integrating with external APIs while preserving Drizzle’s type-safety and identity ergonomics.