Add Balancer V2 indexer DB migrations

[AryanGodara]

Description

First PR of the balancer-v2 indexer stack. Adds the tables the indexer writes pool data into. Migrations only.

Changes

• New migration V111__balancer_v2.sql adds balancer_v2_pools and balancer_v2_pool_tokens to the pool-indexer's per-network DB.
• Checkpoints reuse the existing pool_indexer_checkpoints table — balancer and uniswap-v3 factory addresses are distinct, so they share it without collision.
• Partial index on token WHERE decimals IS NULL for the decimals backfill.

How to test

1. docker compose up -d applies the migration via flyway.
2. Check balancer_v2_pools and balancer_v2_pool_tokens exist in the pool_indexer DB.

[github-actions]

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