[9.5] Add request-converter code generation (#8612)

[flobernd]

Summary

Manual backport of #8612 to 9.5. As with the 9.4 backport (#8927), the branch carries the handwritten changes (CodegenAttribute, partial/[Codegen] markers on handcrafted types, the RequestConverter projects, tests, and build scripts), with the generated files recreated on top by the "Generate Client" workflow (run 29902798736).

Notes

• Regenerating against the 9.5 specification required a generator fix for a union selector crash triggered by the new esql NamedParameterValue type (elastic/elastic-client-generator-net#188, backported to 9.5 via elastic/elastic-client-generator-net#191).
• The same spec change alters the esql params dictionary value type from ICollection to Union, ClassifiedNamedParameter>; the handwritten EsqlQueryExecutor is adapted accordingly.