feat(keyring-eth-hd)!: add legacyEntropySource

[ccharly]

Introducing the new entropy source ID to the v2 HdKeyring.

This will allow us to use it in combination with :withKeyringV2 to filter out HD keyrings with their new deterministic entropy source.

This way we can have mapping between both:

const entropySource = this.messenger.call('KeyringController:withKeyringV2', {
  filter: ({ keyring }) =>
    isHdKeyring(keyring) && keyring.legacyEntropySource === legacyEntropySource,
  ({ keyring }) => keyring.entropySource
);

// OR:
const legacyEntropySource = this.messenger.call('KeyringController:withKeyringV2', {
  filter: ({ keyring }) =>
    isHdKeyring(keyring) && keyring.entropySource === entropySource,
  ({ keyring }) => keyring.legacyEntropySource
);

---

Note

Medium Risk
Breaking rename/repurpose of entropySource affects consumers that assumed the legacy keyring id; migration depends on correctly populating legacyEntropySource.

Overview
Breaking change to v2 HdKeyring entropy identifiers: public entropySource is now the canonical fingerprint entropy:bip44:mnemonic:<uuid> (Bip44MnemonicEntropyId), and the previous KeyringController keyring id moves to optional public legacyEntropySource on HdKeyringOptions for migration-era bidirectional lookup via withKeyringV2.

@metamask/keyring-sdk adds AsEntropyId to narrow string template types to EntropyId at compile time; tests use a real canonical entropy id and cover legacyEntropySource when set vs omitted.

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