Allow enabling the Cost Model in other networks than mainnet

[tilacog]

After the multi-network changes introduced in 668, the Cost Model feature was restricted to Ethereum Mainnet.
We should find a way to enable this feature for other networks besides that.

Originally posted by @fordN in #668 (comment)

[madumas]

If the roadblock is the availability of GRT/DAI uniswap pairs on other network, the agent should still allow using cost models when inject-dai=off.

[fordN]

Gathering some notes from a quick brainstorming session with @tilacog

Tasks to do

• Update code to support cost-models on each network: ensure data models include protocolNetwork, etc...
• Decide and implement network agnostic approach to fetching GRT/DAI pair price

Some initial ideas for network agnostic approaches to fetching price:

• Create a small, specific subgraph tracking the GRT/DAI or GRT/USD pairs on Uniswap (and maybe other protocols too?)
• Simply use the GRT/DAI price from Mainnet across all networks
• ....

[abarmat]

I think that for the purpose of the cost model we could just use the data from Ethereum. What's important is that the oracle we use has enough volume.

[abarmat]

Other ones to investigate: https://docs.mean.finance/v/oracles/ and https://docs.oracles.rip/content/what-is-price.html - also this one for offchain oracles https://github.com/mean-finance/sdk

[alex-pakalniskis]

#899

[alex-pakalniskis]

edgeandnode/gateway#971