eth: Add --historymode flag for configurable blockchain history pruning FIX #31277

[VolodymyrBg]

PR Description

This PR adds a new --historymode command-line flag to geth that allows users to configure how blockchain history is retained.
Fix #31277
#31277

Overview

The --historymode flag provides a way to configure how much of the blockchain history is retained by the node. Currently, there are two options:

• full: Keeps all blockchain history (current default)
• pruned: Keeps only recent history according to StateHistory and TransactionHistory settings
  This flag is conceptually similar to --syncmode and --gcmode, helping users better control disk usage and database size.

Implementation Details

• Added a new HistoryMode union type in eth/ethconfig
• Added the HistoryMode field to the Config struct
• Added the --historymode CLI flag
• Implemented proper handling of the flag value in the configuration
• Added logic to adjust transaction and state history based on the chosen mode
• Ensured compatibility with existing flags like --gcmode

Behavior

When --historymode=pruned is set:

• Transaction history will be limited to the value set by --txlookup (defaults to 2350000 blocks)
• State history will be limited similarly, preventing unbounded growth
• When combined with --gcmode=archive, a warning is shown and the incompatible settings are adjusted
  This PR lays the groundwork for eventually making pruned mode the default, which will help reduce disk usage for regular nodes without sacrificing critical functionality.

Testing

The implementation has been tested with various combinations of flags to ensure proper behavior:

• --historymode=full (default)
• --historymode=pruned
• --historymode=full --gcmode=archive
• --historymode=pruned --gcmode=archive (produces appropriate warnings)
  Tested on a local blockchain to ensure transaction history and state pruning worked as expected.