EVM/Proxy: optional use a rollup node endpoint

Context

It's painful to always have a tezos node and a rollup-node to test the proxy server. Let's use mock values if there is no endpoint.

Manually testing the MR

Run a proxy server without a rollup node endpoint

$ ./octez-evm-proxy-server run -rollup-node-endpoint mockup

Then do the same with a valid endpoint:

$`./octez-evm-proxy-server run --rollup-node-endpoint http://localhost:8888

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR