### Description

In cases where the same agg is used, instead of computing it multiple times, make another reference to it:
```
stats a = min(x), b = min(x) by z
```

should become
```
stats a = min(x) by z
eval b = a
keep a,b,z
```

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
var plan = plan("""
            from test
            | eval x = emp_no + 2
            | eval y = salary + 3
            | keep x, y
            """);
        var project = as(plan, Project.class);
        var eval = as(project.child(), Eval.class);
        assertThat(Expressions.names(eval.fields()), contains("x", "y"));
        var limit = as(eval.child(), Limit.class);
        var source = as(limit.child(), EsRelation.class);
    }
    public void testCombineLimits() {
        var limitValues = new int[] { randomIntBetween(10, 99), randomIntBetween(100, 1000) };
        var firstLimit = randomBoolean() ? 0 : 1;
        var secondLimit = firstLimit == 0 ? 1 : 0;
        var oneLimit = new Limit(EMPTY, L(limitValues[firstLimit]), emptySource());
        var anotherLimit = new Limit(EMPTY, L(limitValues[secondLimit]), oneLimit);
        assertEquals(
            new Limit(EMPTY, L(Math.min(limitValues[0], limitValues[1])), emptySource()),
            new LogicalPlanOptimizer.PushDownAndCombineLimits().rule(anotherLimit)
        );
    }
    public void testMultipleCombineLimits() {
        var numberOfLimits = randomIntBetween(3, 10);
        var minimum = randomIntBetween(10, 99);
        var limitWithMinimum = randomIntBetween(0, numberOfLimits - 1);
        var fa = getFieldAttribute("a", INTEGER);
        var relation = localSource(BlockFactory.getNonBreakingInstance(), singletonList(fa), singletonList(1));
        LogicalPlan plan = relation;