[file progress store] make FileProgressStore not fail on read

[zihehuang]

Description

File progress store fails on read if the provided progress file path does not exist. This PR allows FileProgressStore to load without the progress file present. When save is called, the progress is saved to file as usual.

Test plan

Updated existing test

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Release notes

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• [ ] Protocol:
• [ ] Nodes (Validators and Full nodes):
• [x] Indexer: A user no longer need to pre-create the progress file when using the FileProgressStore in the custom indexer
• [ ] JSON-RPC:
• [ ] GraphQL:
• [ ] CLI:
• [ ] Rust SDK:
• [ ] REST API:

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Name	Status	Preview	Comments	Updated (UTC)
sui-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 16, 2024 8:54pm

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