feat: Custom Base Token Gas Price Oracle

[jrchatruc]

What ❔

This PR adds a new BaseTokenFetcher component to be used by custom base token stacks. It's in charge of caching the latest conversion rate between the base token and eth so gas prices can be converted accordingly (both l1_gas_price and l1_pubdata_price).

A new config toml file is added holding the configuration for it, currently just a host and poll_interval variables to get the host from which the conversion rate is queried and how often the conversion rate is refreshed. The component is initialized by adding base_token_fetcher as a server --component.

There is also an example server component that acts like a sample conversion rate api, it returns a fixed conversion rate of value 42. This component can instantiated by adding the dev_conversion_rate_api argument to the the server components at initialization.
To see a complete the implementations as a whole, run:

zk server --components "api,tree,eth,state_keeper,housekeeper,commitment_generator,base_token_fetcher,dev_conversion_rate_api"

to quickly test out the fetcher component. This will be deleted once proper tests are written.

Disclaimer

As explained here, current implementation is not overflow safe, this is left for a pending continuation of this PR, as of this one, it's stated that there's no u64 safety.

Why ❔

Gas Prices in a custom base token setting need to be in the native currency of the chain. These prices are partially determined by l1 gas prices in L1, which are in eth, so a conversion is required.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zk fmt and zk lint.
• Spellcheck has been run via zk spellcheck.
• Linkcheck has been run via zk linkcheck.

[juan518munoz]

@popzxc how would you recommend we test correct functionality of the added components? Any example we can use as a template?

[popzxc]

@juan518munoz I don't think that there exists a good template that would help testing this feature, though we do have some infra that may help with it.

For instance, we already run tests in a matrix in CI. You can add a value for custom base token there, and when tests are ran for custom token, you may add the relevant config and spawn the server with the dev conversion rate fetcher. An example of such integration in CI could be found in this PR.

That said, I believe that it should be done as a separate PR to not overly extend the scope for this PR.

[fkrause98]

Thanks for the answer @popzxc, we'll be working on those tests on this draft.