fix(codec): fix last field compilation check

[joshieDo]

Fixes a previously modified last field check on compact codec and adds additional context to main_codec

This was probably changed to allow CheckpointBlockRange to be compiled. The necessary changes were introduced, and it doesn't affect the actual encoding of the value (not a db breaking change).

I added a bit more context on the relevant assertion:

 ! Be careful before changing the following assert ! Especially if the type does not
 implement proptest tests.

 The limitation of the last placed field applies to fields with potentially large sizes,
 like the `Transaction` field. These fields may have inner "Bytes" fields, sometimes even
 nested further, making it impossible to check with `proc_macro`. The goal is to place
 such fields as the last ones, so we don't need to store their length separately. Instead,
 we can simply read them until the end of the buffer.

 However, certain types don't require this approach because they don't contain inner
 "Bytes" fields. For these types, we can add them to a "known_types" list so it doesn't
 trigger this error. These types can handle their own deserialization without
 relying on the length provided by the higher-level deserializer. For example, a
 type "T" with two "u64" fields doesn't need the length parameter from
 "T::from_compact(buf, len)" since the length of "u64" is known internally (bitpacked).

[shekhirin]

thank you 🙏
also love the comment explaining the behaviour

[codecov]

Codecov Report

Merging #3543 (9effb54) into main (4f32f56) will decrease coverage by 35.26%.
The diff coverage is 87.50%.

Impacted Files	Coverage Δ
crates/storage/codecs/derive/src/lib.rs	68.10% <ø> (ø)
...rates/storage/codecs/derive/src/compact/structs.rs	81.81% <50.00%> (+0.23%)	⬆️
...tes/storage/codecs/derive/src/compact/generator.rs	98.91% <100.00%> (+0.03%)	⬆️

... and 341 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	?
unit-tests	33.69% <87.50%> (-30.33%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	15.34% <ø> (-7.68%)	⬇️
blockchain tree	29.79% <ø> (-51.47%)	⬇️
pipeline	45.72% <ø> (-41.27%)	⬇️
storage (db)	36.76% <87.50%> (-37.07%)	⬇️
trie	41.92% <ø> (-53.73%)	⬇️
txpool	26.37% <ø> (-24.78%)	⬇️
networking	39.17% <ø> (-38.70%)	⬇️
rpc	15.38% <ø> (-42.50%)	⬇️
consensus	35.92% <ø> (-26.67%)	⬇️
revm	19.57% <ø> (-15.43%)	⬇️
payload builder	4.00% <ø> (-2.84%)	⬇️
primitives	50.25% <ø> (-38.26%)	⬇️

[gakonst]

merging - clippy unrelated

[gakonst]

love it - seems fine to have footgun at codec-related logic, is low level enough and i like that we're highlighting it