Add balancer v2 indexing to pool-indexer

[AryanGodara]

Description

The indexer part for bal-v2. Finds balancer pools from each factory's PoolCreated event and stores their static data, the same shape the subgraph used to serve.

Changes

• For each configured factory, index PoolCreated, then read getPoolId, Vault.getPoolTokens, token decimals, and (weighted pools) getNormalizedWeights.
• A backfill task fills in token decimals that couldn't be read at discovery.
• Move the shared indexer plumbing out of the uniswap-v3 module into indexer/mod.rs so both protocols reuse it.

How to test

• cargo nextest run -p pool-indexer
• run the indexer against a network with balancer factories configured; rows land in balancer_v2_pools and balancer_v2_pool_tokens.

[github-actions]

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