Add multi network support to `graph-network-indexer` chart

[calinah]

v0.20.18 of indexer-agent comes with capacity for the Agent to interact with multiple protocol networks (blockchains that host The Graph Protocol) by running a single instance of the indexer-agent program. We need to adapt chart to be able to support various configs for various chains. See https://github.com/graphprotocol/indexer/releases/tag/v0.20.18