fix: fetch gas-prices from the chain config if provided

[freak12techno]

Fixes #1507

The behaviour should be backwards-compatible:

• if fees.fee_tokens is not empty, we use it
• if it is empty, we fetch assets list and set the denom/gas price from it, if it's present (same way as before)
• if asset list is also empty, we do not set it at all (same way as before)

[freak12techno]

@Reecepbcups can you take a look?

[Reecepbcups]

LGTM