## Why this should be merged
Allows for modification of `types.Body` payload data + RLP encoding
without placing the entire RLP burden on the `libevm` user as we did
with `types.HeaderHooks`.
## How this works
RLP encoding of a struct is simply a concatenation of RLP encodings of
fields, encompassed by an RLP "list". The
`AppendRLPFields(rlp.EncoderBuffer, ...)` hook exploits this and plugs
in before all `rlp:"optional"`-tagged fields to allow for inclusion of
any new fields. The `EncoderBuffer` SHOULD be used as the `io.Writer`
passed when encoding each field: `rlp.Encode(buffer, fieldValue)`.
`Body` doesn't have `{En,De}codeRLP` methods so they are implemented to
identically replicate original behaviour when a no-op hook is present.
This pattern is sufficient for the `ava-labs/coreth` modifications of
`Body` but can be modified / extended for more complex scenarios, like
`Header`.
> [!NOTE]
> This PR does not include registration of the hooks as that was not the
initial goal and adding them would create too much PR bloat. There is a
placeholder `var todoRegisteredBodyHooks` global variable that can only
be set in tests.
## How this was tested
- Backwards compatibility: the new methods are fuzzed against a `type
withoutMethods Body` passed directly to `rlp.{En,De}code()`
- `coreth` compatibility: unit test of a local implementation of
`BodyHooks` demonstrating reproducibility of RLP encoding.
---------
Signed-off-by: Arran Schlosberg <519948+ARR4N@users.noreply.github.com>
Co-authored-by: Quentin McGaw <quentin.mcgaw@avalabs.org>
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