Gas based keeper rewards

[leomassazza]

The objective of this PR is to get the cost of a transaction based on gas price. It is used to dynamically calculate the keeper rewards for settlement, flag and liquidate, and to calculate the required margin to pay the keeper upon liquidation.

It makes use of a node that gets a transaction cost based on current gas price, and then the keeper rewards are added to the cost.

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Detailed TODO list:

• Add mock TxGasPriceOracle
• Add node for keeper rewards gas cost
• Add KeeperCost library to storage that calls the node
• Add node to bootstrap
• Configure node with all zeros (not alter current tests)
• Tests pass with node
• Test not configurable by non-owner
• Test to check revert if not configured
• Add cost of execution on settlement
• Add cost of execution on flag
• Add cost of execution on liquidation
• Add cost of flag/liquidate to isElegibleForLiquidation
• Dynamic min and max keeper reward caps
• Test settlement cost is used on settlement
• Test caps on flag + liquidate
• Test flag / liquidate for 1 margin 1 small position
• Test flag / liquidate for 1 margin 2 small positions (multiple feeds for positions)
• Test flag / liquidate for 1 margin 1 large position (multiple steps)
• Test flag / liquidate for 2 margins 1 small position (multiple feeds for margin)
• Test flag/liquidate cost is added to isElegibleForLiquidation
• Test like Order.marginValidation.test with caps
• Add extra reward for liquidators (execution reward on top of cost)
• On tests, confirm liquidator got the rewards
• review comments and docs and add missing ones
• cleanup (no console logs or dev stuff)
• lint
• all test pass green

[sunnyvempati]

hope the comments make sense. hmu in the AM with any q's. we're very close! great work on this

[sunnyvempati]

couple changes; pre-approving. awesome work on this

[noahlitvin]

Closing in favor of #1904