Plugins & validation & shell: rename Shell_plugin.FILTER

Context

Fixes #5282 (closed)

The protocol plugin for the mempool was initially called Filter because its purpose was to provide pre- and post-filters. But it has a broader scope now. Especially now that it is used in lib_validation, it's weird to build e.g. the Block_validation module from a module called Filter which actually wraps the current protocol and its mempool/validation plugin.

The main change of this MR is to replace

module type FILTER = sig
  module Proto : Registered_protocol.T

  module Mempool : ... (* the signature of proto_xxx/lib_plugin/mempool.mli *)
end

with

module type PROTOCOL_WITH_VALIDATION_PLUGIN = sig
  include Registered_protocol.T

  module Plugin : ... (* the signature of proto_xxx/lib_plugin/mempool.mli *)
end

This means that in block_validation.ml, prevalidation.ml etc. instead of depending on a module Filter that's used as e.g. Filter.Proto.operation and Filter.Mempool.pre_filter, we depend on a module Proto and write Proto.operation and Proto.Plugin.pre_filter.

This MR does no semantic changes.

Manually testing the MR

CI

Checklist

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