[Bug] elastic_kibana_slo.custom_metric should not require "field" when the aggregation type is "doc_count"

[anitahuang-accenture]

Describe the bug
Custom metric SLO doesn't require the "field" field when creating the SLO manually, so the resource should not require "field" when the aggregation type is "doc_count"

To Reproduce
Steps to reproduce the behavior:

1. Create a custom metric SLO with the doc_count type of aggregation and field = "*" or field = null, adding the "filter" field with the desired filter

Expected behavior
SLO created

Debug output
2025-04-01T17:21:46.260-0300 [DEBUG] provider.terraform-provider-elasticstack_v0.11.1: {"statusCode":400,"error":"Bad Request","message":"Failed to validate: \n Excess keys are not allowed:\nbody.indicator.params.good.metrics.field\nbody.indicator.params.total.metrics.field","attributes":{"data":null}}

Versions (please complete the following information):

• OS: [Linux]
• Terraform Version [e.g. 1.6.6]
• Provider version [e.g. v0.11.1]
• Elasticsearch Version [e.g. 8.16.0]