[ES|QL] Change fieldDefinitionType to enum

[stratoula]

Describe the feature:

We can change https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-esql-validation-autocomplete/src/definitions/types.ts#L177 to enum:

export enum FunctionDefinitionType {
  AGG = 'agg',
  SCALAR = 'scalar',
  OPERATOR = 'operator',
  GROUPING = 'grouping',
}

and use them across the codebase.

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)