fjord: Add L2ToL1MessagePasser account storage root to Header `extr…

…aData`

Introduces an addition to the `Fjord` hardfork, where upon activation,
consensus will require L2 block headers to contain the 32 byte account
storage root of the `L2ToL1MessagePasser` predeploy after the block's
execution.

specs/fjord/header.md

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+# Fjord L2 Block Header Changes
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**
+
+- [Header Changes](#header-changes)
+  - [Timestamp Activation](#timestamp-activation)
+  - [Header Validity Rules](#header-validity-rules)
+  - [Header Withdrawals Root](#header-withdrawals-root)
+    - [Rationale](#rationale)
+    - [Forwards Compatibility Considerations](#forwards-compatibility-considerations)
+    - [Client Implementation Considerations](#client-implementation-considerations)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+# Header Changes
+
+After the Fjord hardfork's activation, the L2 block header's `withdrawalsRoot` field will consist of the 32-byte
+[`L2ToL1MessagePasser`][l2-to-l1-mp] account storage root _after_ the block has been executed.
+
+## Timestamp Activation
+
+Fjord, like other network upgrades, is activated at a timestamp.
+Changes to the L2 Block execution rules are applied when the `L2 Timestamp >= activation time`.
+Changes to the L2 block header are applied when it is considering data from a L1 Block whose timestamp
+is greater than or equal to the activation timestamp.
+
+## Header Validity Rules
+
+After Fjord activation, an L2 block header's `withdrawalsRoot` field is valid iff:
+
+1. It is exactly 32 bytes in length.
+1. The [`L2ToL1MessagePasser`][l2-to-l1-mp] account storage root, as committed to in the `storageRoot` within the block
+   header, is equal to the header's `withdrawalsRoot` field.
+
+## Header Withdrawals Root
+
+| Byte offset | Description                                               |
+| ----------- | --------------------------------------------------------- |
+| `[0, 32)`   | [`L2ToL1MessagePasser`][l2-to-l1-mp] account storage root |
+
+### Rationale
+
+Currently, to generate [L2 output roots][output-root] for historical blocks, an archival node is required. This directly
+places a burden on users of the system in a post-fault-proofs world, where:
+
+1. A proposer must have an archive node to propose an output root at the safe head.
+1. A user that is proving their withdrawal must have an archive node to verify that the output root they are proving
+   their withdrawal against is indeed valid and included within the safe chain.
+
+Placing the [`L2ToL1MessagePasser`][l2-to-l1-mp] account storage root in the `withdrawalsRoot` field alleviates this burden
+for users and protocol participants alike, allowing them to propose and verify other proposals with lower operating costs.
+
+### Forwards Compatibility Considerations
+
+As it stands, the `withdrawalsRoot` field is unused within the OP Stack's header consensus format, and will never be
+used for other reasons that are currently planned. Setting this value to the account storage root of the withdrawal
+directly fits with the OP Stack, and makes use of the existing field in the L1 header consensus format.
+
+### Client Implementation Considerations
+
+Varous EL clients store historical state of accounts differently. If, as a contrived case, an OP Stack chain did not have
+an outbound withdrawal for a long period of time, the node may not have access to the account storage root of the
+[`L2ToL1MessagePasser`][l2-to-l1-mp]. In this case, the client would be unable to keep consensus. However, most modern
+clients are able to at the very least reconstruct the account storage root at a given block on the fly if it does not
+directly store this information.
+
+[l2-to-l1-mp]: ../protocol/predeploys.md#L2ToL1MessagePasser
+[output-root]: ../glossary.md#l2-output-root