[go-server] Feat: add required assertions to models

[lwj5]

This PR adds required assertions for models.

• Checks a model if required fields are non-zero (tested)
• Checks a model's parent (recursive) if required fields are non-zero (tested)
• Checks a model's children (recursive) if required fields are non-zero (tested)
• Checks the model in an array (recursive e.g. slice in slice in slice) if required fields are non-zero (tested)
• Works with nullable and non-nullable types (tested)

This PR does not do assertion checks for:

• Map values (if they are a model)

Feel free to give suggestions, feedback!

Fixes:
A pointer of pointer?

	{{paramName}} := &{{dataType}}{}
	if err := json.NewDecoder(r.Body).Decode(&{{paramName}}); err != nil {

I removed the pointer from the first line.

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[lwj5]

Hi @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d, please help to take a look at this PR

[lwj5]

Tests failure unrelated
Plugin org.openapitools:openapi-generator-maven-plugin:5.2.0-SNAPSHOT or one of its dependencies could not be resolved

[lwj5]

I think I might be able to add assertions for Nested arrays (array-in-array).

Edit: Done.

[wing328]

Tested locally and didn't spot any issue.