[indexer alt] - make ClientArgs derive default

[patrickkuo]

Description

Make ClientArg derive Default so we can omit optional fields.

Test plan

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Release notes

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• [ ] Protocol:
• [ ] Nodes (Validators and Full nodes):
• [ ] gRPC:
• [ ] JSON-RPC:
• [ ] GraphQL:
• [ ] CLI:
• [ ] Rust SDK:

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[amnn]

@tpham-mysten ended up introducing a builder pattern which means we no longer need to go down this route!