[BUG] [GO] Unknown newStrictDecoder function with model only

Description

To reproduce:

• generate go files with "client.go" into ".openapi-generator-ignore"

Expected:
Generated sources can be compile as usual

Actual:
It doesn't compile anymore because method "newStrictDecoder" doesn't exist (no client.go file)

Proposal:
move useful function into utils.go file.

Linked Issue : #8491

openapi-generator version

Latest Docker image openapi-generator-cli (generated today : 01/21/22)

OpenAPI declaration file content or url

Every valid openapi spec file

Generation Details

extract from generated source:

// Unmarshal JSON data into one of the pointers in the struct
func (dst *FormTemplateBodyItem) UnmarshalJSON(data []byte) error {
	var err error
	// use discriminator value to speed up the lookup
	var jsonDict map[string]interface{}
	err = newStrictDecoder(data).Decode(&jsonDict)
	if err != nil {
		return fmt.Errorf("Failed to unmarshal JSON into map for the discriminator lookup.")
	}

Steps to reproduce

Just add "client.go" into ".openapi-generator-ignore"

For information, I use generator with theses options:

generate -i "/openapi_spec.yaml" --strict-spec true --additional-properties=useOneOfDiscriminatorLookup=true -p enumClassPrefix=true -g go 

Related issues/PRs

Related to this issue #8491 https://github.com/OpenAPITools/openapi-generator/pull/8491/commits/31555e593f1a3de5d5c7756a80da2c3233029a11 --> commit 31555e593f1a3de5d5c7756a80da2c3233029a11

Suggest a fix

Move useful methods into utils.go

Reproduce generation with useOneOfDiscriminatorLookup=true while adding client.go to .openapi-generator-ignore, then inspect the generated client.go and utils.go files. Confirm the missing newStrictDecoder reference and compile the generated sources; done means generation succeeds without client.go and the output compiles.