feat: integrate HeaderValidator + make FileClient generic over block

[klkvr]

Integrates HeaderValidator from #12648 into ReverseHeadersDownloader. This required a small helper for casting traits (AsHeaderValidator)

Additionaly makes FileClient generic over block getting us closer to abstraction of CLI

[emhane]

scope has grown to #12475, conflicts with #12521 @TropicalDog17

[mattsse]

lgtm, one q

[mattsse]

hmm, isn't this AsHeaderValidator<H> bound redundant if AsHeaderValidator is impl for all HeaderValidator

[klkvr]

doesn't compile without it, unsure why exactly, probably has something to do with the fact that we are invoking this on Arc impl

error[E0599]: no method named `as_header_validator` found for struct `Arc<dyn reth_consensus::Consensus>` in the current scope
   --> crates/stages/stages/src/sets.rs:250:35
    |
250 |                 consensus.clone().as_header_validator(),
    |                                   ^^^^^^^^^^^^^^^^^^^ method not found in `Arc<dyn Consensus>`
    |
   ::: /Users/klkvr/github/reth/crates/consensus/consensus/src/lib.rs:150:8
    |
150 |     fn as_header_validator<'a>(self: Arc<Self>) -> Arc<dyn HeaderValidator<H> + 'a>
    |        -------------------
    |        |
    |        the method is available for `Arc<Arc<dyn reth_consensus::Consensus>>` here
    |        the method is available for `Arc<&Arc<dyn reth_consensus::Consensus>>` here
    |
help: consider wrapping the receiver expression with the appropriate type
    |
250 |                 Arc::new(consensus.clone()).as_header_validator(),
    |                 +++++++++                 +
help: consider wrapping the receiver expression with the appropriate type
    |
250 |                 Arc::new(&consensus.clone()).as_header_validator(),