[staking_pool] Owner can store metadata about their pool

[ilblackdragon]

There are usually few pieces of data about the pool that people are interested in:

• Logo (IPFS hash or url)
• Description
• Website URL (or IPFS hash)

This optionally can leave in a separate registry too.

[evgenykuzyakov]

Let's have a contract for this. This seems like a common problem for multiple contracts. We want to store metadata somewhere and verify signature, but without having this functionality on the contract itself.

If you look at https://github.com/near-examples/token-factory/blob/master/contracts/token/src/lib.rs There I've embedded the token description directly into the contract, but I don't know if that's a good idea. What if another registry wants to support different format. Maybe if we expose the owner_id as a view method, this will allow to verify ownership of the contract. And the owner can set this on variable registries