[core] add gas arg to ext_gas_price (#461)

* add gas argument

* Add gas parameter to ext_gas_price

* Fix up gas_price offchain api

* Fix up env_access api

* Emulate gas price calculation

* Make clippy happy

* Use Saturating impl for emulating gas price calculation offchain

* Fmt

* Add offchain gas_price test

* Make clippy happy
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