Add extensible compression support for RPC requests

[fubhy]

Summary

Implements extensible compression support for Graph Node's outgoing JSON-RPC requests to upstream providers, addressing issue #5671. The implementation uses an enum-based design that makes it easy to add new compression methods in the future without breaking existing configurations.

Key Changes

• Enum-based compression configuration: Replaced boolean compression_enabled field with Compression enum (None, Gzip)
• Per-provider configuration: Each RPC provider can have independent compression settings
• Future-extensible design: Clear pattern for adding Brotli, Deflate, and other compression methods
• Backward compatible: Defaults to no compression, existing configs continue to work
• Type-safe: Prevents invalid compression method strings in configuration

Configuration Format

Enable gzip compression:

[chains.mainnet]
provider = [
  { 
    label = "mainnet-rpc", 
    details = { 
      type = "web3", 
      url = "http://rpc.example.com", 
      features = ["archive"],
      compression = "gzip"
    }
  }
]

Disable compression (default):

compression = "none"  # or omit field entirely

Implementation Details

• Configuration: Compression enum in node/src/config.rs with serde support
• Transport layer: Updated Transport::new_rpc() to handle compression enum with match statement
• Future ready: Commented placeholders show how to add new compression methods
• Testing: Unit tests for both gzip and none compression parsing
• Dependencies: Added gzip feature to reqwest dependency

Future Extensions

Adding new compression methods requires only:

1. New enum variant: #[serde(rename = "brotli")] Brotli
2. Match arm in transport: Compression::Brotli => client_builder.brotli(true)
3. Corresponding reqwest feature in Cargo.toml

Test plan

• Unit tests pass for compression configuration parsing
• Configuration examples validate correctly
• Transport layer handles enum variants properly
• Manual testing with real RPC providers that support gzip
• Performance testing to verify compression benefits
• Integration tests with actual compressed RPC traffic

🤖 Generated with Claude Code

[fubhy]

@lutter This is also building now ...

[lutter]

Rebased to latest master (and cleaned up a little)

[lutter]

I just looked at this, and seeing how we do similar things for other providers, why do we use a separate compression = "gzip" here instead of following the pattern established for firehose where we use features = [ "compression" ]; to keep our options open for other compression methods, we could do something like features = [ "gzip" ] or features = ["compression/gzip"] for RPC providers.

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