feat: allow providing old mnemonics to collect pending TAPv1 RAVs

[tmigone]

Summary

• Add support for legacy operator mnemonics to collect TAPv1 RAVs after operator wallet rotation
• When rotating operators, pending TAPv1 RAVs from allocations created by the old operator can now be collected by
  providing the old mnemonic(s)

Details

TAPv1 RAV collection requires signing with the allocation-specific key derived from the operator mnemonic that created the allocation. After rotating to a new operator wallet, RAVs for old allocations would fail to collect because the new operator's mnemonic cannot derive the correct signing key.

Note that TAPv2 (Horizon) does not require the original operator mnemonic for RAV collection so legacy operator mnemonics are not necessary if keys are rotated.

This PR adds a --legacy-mnemonics option that allows providing previous operator mnemonics. The TapCollector will:

1. Try the current operator wallet first
2. Fall back to legacy mnemonics if the current wallet doesn't match the allocation

Usage

CLI:

--legacy-mnemonics "old mnemonic words..." --legacy-mnemonics "older mnemonic..."

Environment variable (pipe-separated):

INDEXER_AGENT_LEGACY_MNEMONICS="old mnemonic words... | older mnemonic words..."