add TraceableValue typeclass and base implementations

[bpholt]

Introduces a lawless typeclass that is responsible for converting between the domain types used by a traced application and the TraceValue type that is used to track attribute values that can be attached to spans.

Using a typeclass approach instead of relying on implicit conversions has a couple advantages. My original motivation was to allow for a form of semi-automatic instrumentation using cats-tagless's Weave effect, which requires method parameter types all be constrained by a user-specified typeclass. This requirement means that we can generically convert a value of type A: TraceableValue to a TraceValue in positions where the compiler cannot prove that the existing implicit conversions would have worked.

I recently published Dwolla/natchez-tagless to facilitate this kind of semi-automatic instrumentation. (If this PR is merged, I'd modify that library to remove its ToTraceValue typeclass and use the TraceableValue I'm proposing here.)

Another benefit comes with the use of refinement / opaque types, where the typeclass instance can format or redact a value before its inclusion as a span attribute.

[bpholt]

I explored trying to use A: TraceableValue in most places where TraceValue is used in Natchez today, but ultimately concluded that it wasn't worth it. For example, the put and log methods on Span both take varargs of (String, TraceValue) tuples. Replacing that with (String, A) works in the simple case, but we'd have to stop supporting varargs in this way to propagate that change everywhere. e.g. the put in DDSpan.attachError could still work if we created several put methods with increasing arities (or introduced some shapeless-style shenanigans, like using ProductArgs to convert varargs to an HList). That gets trickier when the vararg is used to add an arbitrary list of attributes to the span. For the purposes mentioned above, I concluded it's not worth adding that extra complexity to clean up the TraceValue/TraceableValue distinction.

[bpholt]

Definitely open to bike-shedding over the names here. Right now I think TraceableValue is slightly better than ToTraceValue, which is what I used in our library. 🤷‍♂️