feat: [Indexer] implemented event fetching and processing

[sadiq1971]

Closes #158

[gemini-code-assist]

Summary of Changes

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This pull request introduces a comprehensive event fetching and processing pipeline for the indexer. It establishes a reliable streaming client for the Canton ledger, capable of handling disconnections and authentication issues. A generic framework for decoding and batching events is provided, along with a specific implementation for TokenTransferEvents. The core processing logic ensures atomic persistence of events and updates to derived state, such as token balances and total supply, enabling a robust and consistent indexing solution.

Highlights

• Canton SDK Streaming Client: Implemented a new Canton SDK streaming client (pkg/cantonsdk/streaming/client.go) that provides automatic reconnection with exponential backoff and handles authentication token invalidation for robust ledger event fetching.
• Generic Event Streaming Framework: Introduced a generic Stream[T] type (pkg/cantonsdk/streaming/stream.go) that wraps the core streamer and applies a per-event decode function, allowing for flexible processing of different DAML template types.
• DAML Value Decoding Enhancements: Expanded DAML value decoding utilities (pkg/cantonsdk/values/decode.go) with new functions for robustly extracting text, party, numeric, and timestamp values, including nested and optional fields, to simplify event parsing.
• TokenTransferEvent Decoder: Developed a specialized decoder (pkg/indexer/engine/decoder.go) for CIP56.Events.TokenTransferEvent that filters events based on instrument keys, extracts relevant data, and classifies them as Mint, Burn, or Transfer events.
• Indexer Processor Core Logic: Created the main indexer Processor (pkg/indexer/engine/processor.go) responsible for orchestrating event fetching, atomic persistence of events and derived state (tokens, balances, total supply) in a transactional manner, and includes retry mechanisms for resilient operation.
• New Indexer Data Models: Defined core data structures (pkg/indexer/types.go) such as ParsedEvent, InstrumentKey, Token, and Balance to represent the structured and indexed data derived from Canton ledger events.

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[gemini-code-assist]

Code Review

This pull request introduces a well-structured and robust implementation for fetching and processing events from the Canton ledger. The new streaming package provides a solid foundation with a generic, reconnecting client, and the indexer/engine builds upon it effectively. The code demonstrates good practices like dependency inversion, clear separation of concerns, and thorough testing. My review includes a few suggestions to refactor small pieces of code to enhance clarity and reduce redundancy.

[gemini-code-assist]

The !ev.IsNone("fromParty") check is redundant because ev.OptionalPartyField("fromParty") will return an empty string if the field is None. You can simplify this block.

        if v := ev.OptionalPartyField("fromParty"); v != "" {
			fromPartyID = &v
		}