feat: provider-independent RPC URL templating for registry

[filo87]

User story

As a developer using the Centrifuge registry
I want RPC URL templates with variable substitution to be included in the registry chain configuration
So that apps can support new networks dynamically without requiring app releases, and can use any RPC provider that supports the network

Context

Currently, RPC URLs are hardcoded in the indexer's chains.ts file with specific providers (Alchemy and Quicknode). This creates a limitation where:

• Apps cannot dynamically support new networks based solely on registry data
• Support is restricted to a fixed set of RPC providers
• Adding new networks requires app releases

The registry cleanup removed an RPC URL field that was one of the original goals of the registry system. This field should be re-added with a templating approach to make it truly provider-independent.

Acceptance criteria

• Given a chain configuration in the registry
  When the registry includes RPC URL templates with variable placeholders (e.g., https://{0}.bsc.quiknode.pro/{1}
  Then the template format supports multiple variable substitution patterns

• Given an RPC URL template in the registry
  When apps consume the registry data
  Then they can substitute variables (like API names and keys) at runtime to construct valid RPC URLs

• Given the registry includes RPC URL templates
  When a new network is added to the registry
  Then apps can support that network without requiring a release, assuming one of the major RPC providers supports it

• Given the templating system supports variable arrays (e.g., [API_NAME, API_KEY])
  When different providers use different variable naming conventions
  Then consumers can map their own variable names to the template placeholders

• Given the registry structure is updated
  When the indexer processes registry chains
  Then it can extract and use the RPC URL templates from the registry instead of hardcoded values in chains.ts

Technical considerations

• Template format should support both positional placeholders (e.g., {0}, {1})
• Consider backward compatibility with existing hardcoded RPC URLs during transition
• The templating engine should be simple enough for apps to implement without heavy dependencies
• Template variables should be passed as arrays or objects to allow flexible naming by consumers

Related files

• src/chains.ts - Currently contains hardcoded RPC URLs in the endpoints object (lines 90-124)
• generated/types.ts - Registry type definitions that may need to be updated
• scripts/fetch-registry.mjs - Registry fetching script