feat: implement object pooling for PluginPipeline to improve performance (#146)

# Implement Object Pooling for PluginPipeline

This PR adds object pooling for `PluginPipeline` objects to improve performance by reducing memory allocations and garbage collection overhead. The implementation:

1. Adds a new `pluginPipelinePool` to the `Bifrost` struct
2. Implements methods to get, reset, and release pipeline objects
3. Prewarms the pool during initialization
4. Refactors the request handling code to use pooled pipelines

Additionally, this PR:
- Extracts common validation logic into a `validateRequest` helper function
- Moves utility functions like `providerRequiresKey` and `calculateBackoff` to the utils.go file
- Consolidates the try* methods into a single `tryRequest` function
- Simplifies the request handling flow

These changes should reduce memory pressure and improve performance for high-throughput scenarios.

Author: akshaydeo