feat: allow selecting payload encoding per value

[raszi]

Summary

Allows callers to explicitly select which payload converter (nippy, json, or null) is used when serializing a value, instead of relying on the default best-match selection. This is useful when interoperating with non-Clojure Temporal workers that expect a specific encoding.

Changes

• temporal.converter.encoding (new): introduces a Tagged record and with helper for annotating a value with a target encoding (:null, :nippy, :json, or a raw encoding string like "json/plain").
• temporal.converter.default:
  • create now returns a proxy over DefaultDataConverter that intercepts toPayload and routes Tagged values to the requested converter, falling back to default behavior otherwise.
  • Overrides withContext to propagate SerializationContext to a fresh proxy while reusing the original converters/failure-converter via reflection into PayloadAndFailureDataConverter's private fields.
  • Throws DataConverterException with a clear message when the requested encoding has no registered converter, or when the registered converter can't encode the value.
• temporal.client.core: adds an encode helper that runs values through the client's configured DataConverter.
• temporal.converter.optional: removes unused optional?.
• Tests: expand default_test to cover tagged values across :null/:nippy/:json (both keyword and raw string forms), withContext propagation, and error paths for unregistered/unencodable values.

Test plan

• lein test passes
• Round-trip a (encoding/with x :json) value through a workflow and confirm the resulting Payload metadata reports json/plain
• Confirm untagged values still serialize via the default nippy path (no regression)
• Confirm withContext produces a converter that still honors tagged values

[dotemacs]

This is already used in production, so yes, approving.