cmd,internal/era: implement export-history subcommand (#26621)

* all: implement era format, add history importer/export

* internal/era/e2store: refactor e2store to provide ReadAt interface

* internal/era/e2store: export HeaderSize

* internal/era: refactor era to use ReadAt interface

* internal/era: elevate anonymous func to named

* cmd/utils: don't store entire era file in-memory during import / export

* internal/era: better abstraction between era and e2store

* cmd/era: properly close era files

* cmd/era: don't let defers stack

* cmd/geth: add description for import-history

* cmd/utils: better bytes buffer

* internal/era: error if accumulator has more records than max allowed

* internal/era: better doc comment

* internal/era/e2store: rm superfluous reader, rm superfluous testcases, add fuzzer

* internal/era: avoid some repetition

* internal/era: simplify clauses

* internal/era: unexport things

* internal/era,cmd/utils,cmd/era: change to iterator interface for reading era entries

* cmd/utils: better defer handling in history test

* internal/era,cmd: add number method to era iterator to get the current block number

* internal/era/e2store: avoid double allocation during write

* internal/era,cmd/utils: fix lint issues

* internal/era: add ReaderAt func so entry value can be read lazily

Co-authored-by: lightclient <lightclient@protonmail.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>

* internal/era: improve iterator interface

* internal/era: fix rlp decode of header and correctly read total difficulty

* cmd/era: fix rebase errors

* cmd/era: clearer comments

* cmd,internal: fix comment typos

---------

Co-authored-by: Martin Holst Swende <martin@swende.se>

core/blockchain_reader.go

@@ -410,6 +410,11 @@ func (bc *BlockChain) TrieDB() *trie.Database {
 	return bc.triedb
 }
 
+// HeaderChain returns the underlying header chain.
+func (bc *BlockChain) HeaderChain() *HeaderChain {
+	return bc.hc
+}
+
 // SubscribeRemovedLogsEvent registers a subscription of RemovedLogsEvent.
 func (bc *BlockChain) SubscribeRemovedLogsEvent(ch chan<- RemovedLogsEvent) event.Subscription {
 	return bc.scope.Track(bc.rmLogsFeed.Subscribe(ch))