* [Go] Fix deepObject serialization that are anyOf
Updates the go client generator to have the generated struct for anyOf types conform to `MappedNullable` interface.
Fixes query params serialization for deepObjects that are of `anyOf` type.
Implements the suggestion in https://github.com/OpenAPITools/openapi-generator/issues/19085
* check parameter style for deepObject serialization
* generate samples for go-echo-external-refs-test
* move test back to http port
* restrict to anyof models with discriminator
* update test
* add some tests
added some tests to `modules/openapi-generator/src/test/resources/3_0/go/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml` and regenerated the samples
* [Golang] Move utility functions from client.mustache to utils.mustache
* re-generate Go samples
* re-generate Go samples again
* re-generate samples once more
* update go samples
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Co-authored-by: Martin Lakov <martin.lakov@ocado.com>
