EVM Node: Expose the maximum time between two blocks via a RPC

We introduce GET /evm/time_between_blocks in order for observer nodes to (recursively) get the maximum time between two blocks from the sequencer. As a consequence, we can deprecate the time_between_blocks field of the observer configuration. As a consequence, the detection of stalled streamed RPCs becomes the default behavior for observer nodes, while it was not the case before.

In order to avoid an unnecessary breaking changes from operators perspective, we keep the field time_between_block as part of the encoding definition of the configuration type, but it is being fully ignore.

Manually testing the MR

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.
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