[FEATURE]: Optional joins

### Describe what you want

Similar to the `where` function, I would like to be able to join optionally, based on a condition.

```ts
db
.select()
.from(table)
.where(condition ? eq(...) : undefined)
```

Proposed api (but of course it can be anything else that makes sense and allows for optional joining):

```ts
db
.select()
.from(table)
.leftJoin(anotherTable, condition ? eq(...) : undefined)
```