ESQL: Block loader for pushing LENGTH (#137217)

Creates a `BlockLoader` for pushing the `LENGTH` function down into the
loader for `keyword` fields. It takes advantage of the terms dictionary
so we only need to calculate the code point count once per unique term
loaded.

This `BlockLoader` implementation isn't plugged into the infrastructure
for emitting it because we're waiting on the infrastructure we've
started in #137002. We'll make a follow up PR to plug this in.

We're doing this mostly to demonstrate another function that we can push
into field loading, in addition to the vector similarity functions we're
building in #137002. We don't expect `LENGTH` to be a super hot
function. If it happens to be then this'll help.

Before we plug this in we'll have to figure out emitting warnings from
functions that we've fused to field loading. Because `LENGTH` can emit a
warning, specifically when it hits a multivalued field.

Author: nik9000

server/src/main/java/org/elasticsearch/index/mapper/BlockLoader.java

@@ -13,6 +13,7 @@
 import org.apache.lucene.index.SortedDocValues;
 import org.apache.lucene.index.SortedSetDocValues;
 import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.common.breaker.CircuitBreakingException;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.Releasable;
 import org.elasticsearch.index.mapper.blockloader.docvalues.BlockDocValuesReader;
@@ -377,6 +378,13 @@ interface Docs {
      * also a test implementation, but there may be no more other implementations.
      */
     interface BlockFactory {
+        /**
+         * Adjust the circuit breaker with the given delta, if the delta is negative, the breaker will
+         * be adjusted without tripping.
+         * @throws CircuitBreakingException if the breaker was put above its limit
+         */
+        void adjustBreaker(long delta) throws CircuitBreakingException;
+
         /**
          * Build a builder to load booleans as loaded from doc values. Doc values
          * load booleans in sorted order.
@@ -486,6 +494,12 @@ interface BlockFactory {
          */
         Block constantBytes(BytesRef value, int count);
 
+        /**
+         * Build a block that contains {@code value} repeated
+         * {@code count} times.
+         */
+        Block constantInt(int value, int count);
+
         /**
          * Build a reader for reading {@link SortedDocValues}
          */