Enhance Connection Strategy, ENR Validations and add Database Documentation

[MatheusFreixo]

Overview

This PR bundles three related sets of changes:

1. Dynamic backoff & fork support
2. Stricter ENR fork-digest filtering & validation
3. Comprehensive crawler database documentation

Feature: Dynamic backoff for connection attempts

• dynamicBackoff: Implements an exponential retry delay, adapting base delays by error type.
• Deprecation window extended from 3 hours to 1 month to reduce premature peer pruning.
• Fork support: Adds “Electra” fork digest to the ForkDigests map.
• ISP cleanup: Introduces CleanISP to normalize ISP strings before database insertion.

Feature: Improve ENR fork digest filtering & validation

• DV5 service (dv5_service.go)

  • Only invoke GetForkDigest() when FilterDigest != "all".
  • Skip nodes lacking valid Eth2 data instead of panicking.
  • Enhance trace logs on digest mismatches or missing data.

• Ethereum local node (ethereum/enode.go)

• Add constants for expected hex lengths (ETH2_DATA_HEX_LENGTH, etc.).

• Strict normalization & validation of fork-digest and attnets hex inputs.

• Return errors on incomplete/malformed entries; warn on non-standard lengths.

• ENR parsing (ethereum/enr.go & entries.go)

  • Introduce a structured ParseError to capture ENR parse failures.
  • Track presence and parse errors for both Eth2Data and Attnets on EnrNode.
  • Provide helpers HasValidEth2Data() and GetForkDigest().
  • Enforce a fixed 16-byte size for eth2 data, with clear error reporting.
  • Generalize CountBits() to safely handle variable-length bitfields.

Feature: Add crawler database documentation

• New file database.md fully describes all tables:
  • peer_info, eth_nodes, eth_status, eth_blocks, eth_attestations, conn_events, active_peers, ips
• Details columns, types, defaults and indexes.
• Clarifies differences between ENR discovery vs. successful connections (eth_nodes vs. eth_status).