Add `apiClassPrefix` / `apiClassSuffix` configuration options

[halotukozak]

Description

Add apiClassPrefix (String, default "") and apiClassSuffix (String, default "Api") options to JustworksSpecConfiguration. Currently the suffix "Api" is hardcoded in ClientGenerator.kt:52. Users should be able to customize generated class names (e.g. PetstoreClient instead of PetstoreApi).

Acceptance criteria

• JustworksSpecConfiguration DSL supports apiClassPrefix and apiClassSuffix
• Generated client class names use configured prefix/suffix
• Default suffix is "Api" — no breaking change
• Tests cover custom prefix, custom suffix, and both combined
• README documents the options