[jdjioe5-cpu]

PR: feat(cli): add stellar ledger lookup subcommand to convert between timestamp and ledger sequence (#2285)

Summary

Resolves #2285.

Adds a new stellar ledger lookup (with alias convert) subcommand under stellar ledger that enables bidirectional conversion between timestamps (closed at) and ledger sequence numbers:

1. Timestamp to Ledger Sequence (--timestamp <TIMESTAMP>):
   • Accepts Unix epoch timestamps in seconds (or milliseconds) or RFC3339 / ISO-8601 strings (e.g. 2024-05-01T12:00:00Z).
   • Retrieves the available ledger boundary (oldest_ledger to latest_ledger) from the network RPC.
   • If the target timestamp is newer than or equal to the latest ledger, returns the latest ledger.
   • If the target timestamp is within the available RPC history window, performs an accelerated binary/interpolation search to precisely find the ledger closed at or immediately before the target time ("the closest ledger that happened just before the time").
   • If the target timestamp is older than the oldest ledger retained by the RPC, returns a descriptive error indicating the available retention range.
2. Ledger Sequence to Timestamp (--seq <SEQ>):
   • Queries the specified ledger from the network RPC and returns its formatted closed-at time (RFC3339), Unix timestamp (seconds), and block hash.
3. Structured & Human-Readable Output:
   • Supports --output text (default), --output json, and --output json-formatted.
   • Text output displays Ledger Sequence, Closed At, Unix Timestamp, and Hash.

Changes

• cmd/soroban-cli/src/commands/ledger/lookup.rs: Implements Cmd, LookupResult, timestamp parser, interpolation midpoint calculation, and find_closest_ledger_before_or_at.
• cmd/soroban-cli/src/commands/ledger/mod.rs: Registers Cmd::Lookup (with alias convert) and Error::Lookup.
• Added 8 unit tests in cmd/soroban-cli/src/commands/ledger/lookup.rs covering:
  • Unix seconds and milliseconds parsing
  • RFC3339 / ISO-8601 string parsing with timezone offsets
  • Invalid format rejection
  • Clap argument parsing and mutual exclusion (--timestamp vs --seq)
  • Interpolated search ledger matching logic (find_closest_ledger_before_or_at)
  • JSON serialization

Verification

• cargo test --package soroban-cli --lib commands::ledger: 9 passed (8 new lookup tests + 1 existing)
• cargo clippy --package soroban-cli -- -D warnings: 0 warnings
• cargo fmt --check: Clean formatting