Etherlink/Tezt: wrap sequencer in RPC for multichain

What

This MR modifies the tests in evm_sequencer.ml to wrap the sequencer node in an RPC node, conditionally on the value of the enable_multichain feature flag. When this flag is enabled, the tests run with the wrapped sequencer. Otherwise, they use the standard sequencer node.

It also modifies the sequencer manually created in the multichain test test_make_l2_kernel_installer_config to wrap the sequencer, as it also follows under the category of enabled_multichain but does not use the setup_sequencer function.

Why

The goal is to restrict direct sequencer RPC calls in the multichain setup, requiring some requests to be handled via the RPC node instead in some of our tests. Concretely, this is necessary for modifications introduced in !16906 (closed) to not break all tests.

Manual Testing

Run the following command to manually test the changes:

dune exec etherlink/tezt/tests/main.exe -- alpha sequencer multichain_enabled /dal /threshold_encryption /mainnet /ghostnet /flaky

The CI also runs the tests in tezt_slow.