Feature: `--datadir.static`

# Rationale

E3 can place parts of snapshot and temp onto slow disk.

Doing so with symlinks is anywhere from impossible to extremely awkward when running in Docker, depending on exact volume implementation

A `--datadir.static` (bikeshed exact parameter name to heart’s content) that points to where the slow storage is would be an elegant solution. Similar to Geth ancient and Reth static parameters.

# Implementation

Would need to change the strict “all under datadir” parts of the code, introduce a second variable for slow storage, and default it to datadir.

From there the structure remains as is.