Propagate Request Headers through API Client

[mgyucht]

Changes

Based on changes in databricks/databricks-sdk-go#572.

This PR adds initial support for headers in ApiClient.java. The *Impl classes, when making a request, will construct a map of headers to be included in the request. These are passed through a new parameter in each of GET/PUT/POST/PATCH/DELETE.

In the future, to support dynamic headers, we can modify our codegen template to add those fields to the headers map before calling the ApiClient.

Alternatives considered:

• Adding headers in request objects directly: this would not require any change in the ApiClient interface. Instead, we would introduce a new annotation for headers, much like what we do for query parameters, and reflectively scan the fields of each request object. We could do this, but for headers that have a fixed value (like Content-Type and Accept), these fields would essentially be private final fields that we could iterate through. Users can never interact with these fields, so it seems a bit unusual to have them in the request structure, a public interface.

Tests

[nfx]

the diff seems to be larger than it should be. did you try solving it with method overloading?

Suggested change

	{{- if .Response.ArrayValue -}} return apiClient.getCollection(path, null, {{template "type" .Response.ArrayValue}}.class, headers);
	{{- if .Response.ArrayValue -}} return apiClient.getCollection(path, null, {{template "type" .Response.ArrayValue}}.class{{if .HasRequestHeaders}}, headers{{end}});

[mgyucht]

Some old comment?

For simplicity, we will always pass headers to the ApiClient.