`@ensnode/datasources` — re-implement `mergeAbis` to avoid `ponder/client` dependency

[lightwalker-eth]

Related comment: https://github.com/namehash/ensnode/pull/1978/files#r3132344114

More broadly, it seems that @ponder/client is imported by a number of our apps / packages currently, but it looks to me like it is only still needed inside the ponder-subgraph package?

Ideally we can constrain any use of @ponder/client except for within the ponder-subgraph package.

[coderabbitai]

🔗 Related PRs

#1726 - deps: update ponder to latest patch [merged]
#1778 - ENSDb SDK: group database schema objects [merged]
#1807 - Skip ENSDb Writer Worker validation for ENSIndexer Public Config while in dev mode [merged]
#1851 - Use static import for ENSIndexer Schema used with subgraphGraphQLMidlleware function [merged]

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