feat: add LDValue_SerializeJSON C binding

[cwaldren-ld]

This adds a new API to the LDValue interface for serializing to a JSON string.

Example:

LDValue foo = LDValue_NewBool(true);
char *json = LDValue_SerializeJSON(foo);
// json is the string 'true'
LDMemory_FreeString(json);
LDValue_Free(foo);

It's easier to view this PR commit to commit. The first one was necessary in order to access the boost::json tag_invoke implementations in common, where LDValue is implemented. Luckily, I only needed to pull over the value headers and some utilities.

One thing to note is that boost serializes numbers using scientific notation (e.g. 17 serializes to 1.7E1). Kind of an odd choice, but it is to JSON spec.

BEGIN_COMMIT_OVERRIDE
refactor: move json_errors, primitives, and value from internal to common lib
feat: add LDValue_SerializeJSON C binding
END_COMMIT_OVERRIDE

[cwaldren-ld]

This header wasn't necessary.

[kinyoklion]

I am concerned about the ultimate destination of the header moves. Specifically I am concerned that it may make boost a part of our public API. I am down with providing serialization methods, but not with exposing the serialization implementation.

[cwaldren-ld]

@kinyoklion, I've moved the serialization headers to /detail/serialization in the common lib.