get rid of Coins, use FT instead

[telezhnaya]

While solving one more issue of Coin interface, I've decided to get rid of this concept and use FT instead.

What does it mean for the user:

• We planned to serve FTs and MTs together: we no longer want to do that. So, they need to query FTs and MTs separately. The truth is, for now, we don't have MT support anyway. When MT support will be ready, we will consider making additional endpoints and combining all of them together.
• "Get Coin Balances" had both FT and native NEAR in the response. Now we have 2 separate endpoints for that. Let's go with this solution and add one more endpoint later if users ask for it.
• The paths changed a bit, but, since we don't have active users for now, I'm OK with bringing breaking changes

[telezhnaya]

What I want to be reviewed here:

• New endpoints list. There were NFT and Coins previously, now it looks this way:

- Paths. Unfortunately, github can't build diff, you need to look at 3 files below. Before: https://github.com/near/near-enhanced-api-server/blob/main/src/modules/coin/mod.rs After: https://github.com/near/near-enhanced-api-server/blob/no_coins/src/modules/ft/mod.rs https://github.com/near/near-enhanced-api-server/blob/no_coins/src/modules/native/mod.rs

[telezhnaya]

We had a call with @khorolets , finalised the interfaces and I've also added #53