provider: add in gorli support

[tynes]

Description

This adds in the url for the public ropsten node so that the provider can be instantiated like:

const provider = new OptimismProvider('gorli')

This would set the hostname as rinkeby.optimism.io.

Also notes this in the README.md

Questions

• What is the actual url that is going to be used?

Metadata

Contributing Agreement

• I have read and understood the Optimism Contributing Guide and Code of Conduct and am following those guidelines in this pull request.

[tynes]

Noting the CI failure here:

microservices_1      | 2020-09-16T20:29:41.364Z error:service-entrypoint L1 node returned block number undefined when 0 was expected. Aborting deploy!
microservices_1      | (node:64) UnhandledPromiseRejectionWarning: Error: L1 node returned block number undefined when 0 was expected. Aborting deploy!
microservices_1      |     at deployContractsAndValidateDeployment (/server/packages/rollup-services/build/src/exec/services.js:228:15)
microservices_1      |     at processTicksAndRejections (internal/process/task_queues.js:86:5)
microservices_1      | (node:64) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 30)
microservices_1      | (node:64) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

edit: The CI appears to have failed here due to a timeout.

[tynes]

Updated to use gorli

[karlfloersch]

I think probably best to change it back to Goerli but then after that I think this is good to go! Merrrrge!