[Golang][Client] Fix collectionFormat=multi request bug

Created by: grokify

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@antihax @bvwells @kemokemo

Description of the PR

This PR fixes parameters which use collectionFormat=multi so that duplicate keys (but different values) are sent in the query string instead of a single key with a single CSV concatenated value.

See description of bug here:

https://github.com/OpenAPITools/openapi-generator/issues/3386

Notes

This PR doesn't introduce any breaking changes and is built on the existing antihax/optional.Interface type for an optional []string.

For a breaking change, it would be nice to implement an []string in the optional structs and only use if the array has one or more elements.

For example, the following is currently generated:

type APIOpts struct {
	Name      optional.String
	UserIds   optional.Interface
}

It may be better to generate:

type APIOpts struct {
	Name      optional.String
	UserIds   []string
}