feat: let users pass sync/async functions for resolving option values for headers/params

[KyleAMathews]

Fix #2181

This PR adds support for function-based options in the TypeScript client's params and headers. Functions can be either synchronous or asynchronous and are resolved in parallel when needed.

const stream = new ShapeStream({
  url: 'http://localhost:3000/v1/shape',
  params: {
    table: 'items',
    userId: () => getCurrentUserId(),
    filter: async () => await getUserPreferences()
  },
  headers: {
    'Authorization': async () => `Bearer ${await getAccessToken()}`
  }
})

Common Use Cases

• Authentication tokens that need to be refreshed
• User-specific parameters that may change
• Dynamic filtering based on current state
• Multi-tenant applications where context determines the request

Design Decision

We chose to implement this using direct function values rather than a middleware/interceptor pattern (as seen in libraries like Axios or ky) for several reasons:

• Simplicity: Direct function values in config objects are more intuitive than middleware chains. There's a clear 1:1 relationship between the option and its value resolution.
• Granularity: Each param/header can be individually dynamic. No need to set up middleware just to make a single value dynamic.
• Performance: All async functions are resolved in parallel by default, which isn't always the case with sequential middleware chains.
• Focused API Surface: As we're not a general-purpose HTTP client, we don't need the complexity of a full middleware system. This approach keeps our API surface smaller and more focused on Electric's specific needs.

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[thruflo]

Brilliant -- the usage and docs are both 👌

[KyleAMathews]

Brilliant -- the usage and docs are both 👌

Gotta thank my colleague Windsurf for these 😎

[msfstef]

Looks good! I'm a bit weary of adding async steps in the tight loop, and would like more protection (at both the type and runtime level) agains people setting reserved params and headers