ESQL: Make there be one `commonType`

[nik9000]

Description

There are currently three methods called commonType:

• EsqlDataTypeRegistry.INSTANCE.commonType
• EsqlDataTypeConverter.commonType
• DataTypeConverter.commonType

These seem to exist because we were inheriting from QL's common infrastructure. But we're not any more so we can collapse these.

Also! There are clauses in DataTypeConverter.commonType that don't feel like they make sense in ESQL - or maybe they do, but I don't understand them. We seem to promote keyword to text and strings to numerics. We also have this which feels redundant:

        if (isDateTime(left) && isDateTime(right)) {
            return DATETIME;
        }

It'd be lovely if someone could flatten these into one call and remove the clauses that don't apply to ESQL.

[elasticsearchmachine]

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