Expose function to add entropy tables to pre-built dictionary.

Expose function to add entropy tables to pre-built dictionary.

In some cases a custom dictionary building algorithm tailored for a specific
input can be more effective than the one produced by `ZDICT_trainFromBuffer`,
but with the current API it's not possible encode the entropy tables into the
custom-built dictionary.

This commit extracts the logic to add entropy tables to a dictionary from
`ZDICT_trainFromBuffer` and exposes it as a function
`ZDICT_addEntropyTablesFromBuffer`.