Generate withOptions(Consumer<Builder>) returning a new immutable client

[OmarAlJarrah]

Problem

There's no ergonomic way to derive a lightly-reconfigured client. This depends on having a single cloneable client config (we currently have split config with no toBuilder()).

Proposed change

First decide whether to introduce a single cloneable client-config with toBuilder(). If yes, generate withOptions(Consumer<Builder>) returning a new immutable client. If we keep split config, close this.

Acceptance

• Decision on unified client config
• withOptions if config supports it

Dependencies

• Related Emit a Spring Boot starter per generated API #69 — the Spring starter consumes the client-config / customizer decided here

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Priority: medium · Effort: medium