`NumericSpec` and operations on numeric

[mversic]

          I think it's better to remove the validation and mention in the docs that mul and div can cause the numeric spec to change, and that users need to do their own validation as necessary. This impl assumes too much, there are multiple ways to handle it (round to `self`, round to provided `spec`, return none if accuracy goes out of bounds, etc)

What are the intended use cases of these ops?

Originally posted by @nxsaken in https://github.com/hyperledger/iroha/pull/4976#discussion_r1717161788