Reduce allocations in RpcWorkerConfigFactory.AddProviders

[kshyju]

resolves #10931

1. Replaced Newtonsoft.Json with System.Text.Json for deserializing worker profiles.
2. Previously, all worker configs were loaded and parsed into RpcWorkerDescription objects before filtering using ShouldAddWorkerConfig. Now, we skip loading configs for non-matching runtimes and only parse the config that matches FUNCTIONS_WORKER_RUNTIME, reducing unnecessary overhead after specialization.

This change reduced allocations in the AddProviders method, during specialization (and even during placeholder mode).

Before

Function Name	Total (Allocations)	Self (Allocations)	Total Size (Bytes)	Self Size (Bytes)
RpcWorkerConfigFactory.GetConfigs()	5,618	6	536,320	192
RpcWorkerConfigFactory.AddProviders()	5,039	2	481,990	64
RpcWorkerConfigFactory.AddProvider(string)	4,997	30	476,358	1,040

After

Function Name	Total (Allocations)	Self (Allocations)	Total Size (Bytes)	Self Size (Bytes)
RpcWorkerConfigFactory.GetConfigs()	3,961	6	271,511	192
RpcWorkerConfigFactory.AddProviders()	3,521	2	243,522	64
RpcWorkerConfigFactory.AddProvider(string)	3,479	16	237,890	592

Delta

• RpcWorkerConfigFactory.AddProviders() - Total allocations dropped by 1,518 (30.13%).
• RpcWorkerConfigFactory.AddProviders() - Total allocated size reduced by ~238 KB (nearly 50%).

Before

After

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
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• I have added all required tests (Unit tests, E2E tests)

Additional information

Additional PR information

[liliankasem]

minor comments but lgtm