Adjust Motoko E2E testing workflow to include providers with API keys

[rvanasa]

Context (PR code review thread).

As part of the recent changes to the EVM RPC canister, we want to rework the Motoko E2E tests to cover RPC providers which require an API key without including the test keys in the codebase. This is primarily intended for testing on the ICP mainnet, although we would also like to make this available locally and through CI if possible.

Another closely related goal is to provide a convenient workflow for developers to add their own API keys for local development.