chore: condition export of generate_blob_sidecar on its cfg

[Rjected]

Fixes the following warning when running cargo test -p reth-primitives test_blob_transaction_decode:

warning: unreachable `pub` item
   --> crates/primitives/src/transaction/sidecar.rs:495:1
    |
495 | pub fn generate_blob_sidecar(blobs: Vec<Blob>) -> BlobTransactionSidecar {
    | ---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates
    = note: requested on the command line with `-W unreachable-pub`