ESQL: standardize writeTo method across Functions

[costin]

Description

The vast majority of ESQL functions follow the same pattern, accepting one or two fields and thus end up with the same writeTo method:

    public void writeTo(StreamOutput out) throws IOException {
        Source.EMPTY.writeTo(out);
        for (Expression child : arguments()) {
              out.writeNamedWritable(child);
        }
    }

Yet because the method is not already implemented, a lot of functions end up implementing this method, causing small variants.
This was partially addressed in AggregateFunction however the approach should be generalized to all Functions by:
a. implementing a default final writeTo method
b. (optionally) provide an empty extraWriteTo method (or similar) to allow extra payload to be written - though we'd have to identify a case for this first.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[idegtiarenko]

This should also somehow handle optional arguments in Substring, DateFormat, DateParse and possibly other implementations.
Such arguments are currently written with writeOptionalNamedWriteable. This needs to be preserved if we want to avoid transport change:

elasticsearch/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/Substring.java

Lines 74 to 96 in c3f4f4e

	) {
	super(source, length == null ? Arrays.asList(str, start) : Arrays.asList(str, start, length));
	this.str = str;
	this.start = start;
	this.length = length;
	}
	private Substring(StreamInput in) throws IOException {
	this(
	Source.readFrom((PlanStreamInput) in),
	in.readNamedWriteable(Expression.class),
	in.readNamedWriteable(Expression.class),
	in.readOptionalNamedWriteable(Expression.class)
	);
	}
	@Override
	public void writeTo(StreamOutput out) throws IOException {
	source().writeTo(out);
	out.writeNamedWriteable(str);
	out.writeNamedWriteable(start);
	out.writeOptionalNamedWriteable(length);
	}