[datadog_observability_pipeline] Add processor groups instead of standalone processors

[vladimir-dd]

Replaces standalone processors with processor groups.

Before:

processors {
      filter {
        id      = "filter-1"
        include = "service:my-service"
        inputs  = ["source-1"]
      }
      parse_json {
        id      = "parser-1"
        include = "service:my-service"
        field   = "message"
        inputs  = ["filter-1"]
      }
}

After:

processors {
      processour_group {
        id = "group-1" 
        include = "*"
        inputs  = ["source-1"]
        enabled = true
        
        processor {
          id      = "filter-1"
          include = "service:my-service"
          enabled = true
          filter {}
        }
        
        processor {
          id      = "parser-1"
          include = "service:my-service"
          enabled = true
          parse_json {
            field   = "message"
          }
        }
      
}

Important context:

• individual processor's don't have inputs anymore, only processor groups;
• processor block is required to maintain the order of processors(TF tracks the order only for the blocks of the same type). See TestAccDatadogObservabilityPipeline_multipleProcessorGroups as an example test with reordering.
• required enabled field is added to both processors and processor groups;
• processor blocks(filter, parse_json etc.) must be ListNestedBlock with the size validation(<=1), because of the way TF handles optional blocks. If we use SingleNestedBlock(which makes more sense) for an optional block, which contains any required field, TF considers this block as required. This is a recommended workaround(see Disallow use of SingleNestedBlock in resource schemas hashicorp/terraform-provider-aws#35813 for example).

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