Add lazy-transformer to KeyClassifier for deferred header decoding

[amarziali]

What does this do

Adds a default generic method to AgentPropagation.KeyClassifier:

default <T> boolean accept(String key, T value, Function<T, String> transformer)

The default applies the transformer eagerly and delegates to accept(String, String), so existing classifiers need no changes.

Motivation

Some carriers store header values in raw form (e.g. byte[]) and have to convert them to strings before calling accept. Right now that conversion happens for every header, including the ones the classifier immediately ignores.

An example is kafka that's decoding into base64 all the headers without knowing if it's needed.

This overload lets callers pass the raw value and a conversion function. A classifier that rejects unknown keys early never pays for the conversion at all.

The concrete follow-up is the Kafka TextMapExtractAdapter, where Base64-decoding all message headers is wasteful and currently throws IllegalArgumentException on non-Base64 input. That fix comes in a separate PR.

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Jira ticket: [PROJ-IDENT]

[sarahchen6]

Cool!