Centralize several serde constants

[tisonkun]

... perhaps to the datasketches::codec module, or a new internal consts/serde module.

This follows what @proost does in apache/datasketches-go#97 that I think a good way to go.

So far, we store these constants in datasketches::<sketch>::serde path but we'd better manage them (at least the family IDs) in a centralized place.

[leerho]

So far this list of Family IDs is maintained in DS-Java/.../Common. But I agree, we need a more centralized, independent of language place to keep it. Suggestions?

[tisonkun]

So far this list of Family IDs is maintained in DS-Java/.../Common. But I agree, we need a more centralized, independent of language place to keep it. Suggestions?

Ah .. This issue is intended to keep the list of Family IDs in a centralized place within datasketches-rust. So far, they are scattered in each sketch's module.

Moving to a language-independent place could be another topic to discuss. I'm yet to consider it.

[leerho]

This is very similar to the language independent place for managing "cross-language workflows/scripts".
Issue 10

If we don't have a central place to manage these assignments, they will very easily get out-of-sync.