fix: Update ext-node.toml file at init

[toni-calvin]

This PR updates the code executed in the infrastructure/zk/src/init.ts to update the contents of the ext-node.toml setting the l1_batch_commit_data_generator_mode to Validium when is initialized in validium mode.
It also creates a struct of ConfigLine to keep track of al the necessary data to position itself inside the config file.

How to test

In rollup mode: zk clean --all && zk && zk init should only modify the chain.toml updating the values from floating points to integers (not relevant change, only how code is formatted)
In Validium mode: zk clean --all && zk && zk init --validium-mode should modify the chain.toml, eth_sender.toml, and ext-node.toml. The last one should add a line just after the setted section in the ConfigLine parameter.

If runned in rollup mode after a validium mode execution, the files should look the same as the actual state (just formatting the floating point numbers to integers)

To test is not necessary to make all the initialization, once the Updating mode configuration (first and second step of the init) is finished you can actually see the expected result.