feat: support multiple configurable providers

[jesson1]

problem

sdk only instantiates a single Configurable by default, so adding new named pipeline functions typically requires forking or changing SDK internals — slow and hard to maintain.

solution

Introduce a configurables collection and let runtime users register their own Configurable-style providers (factories). This lets operators/apps add custom pipeline functions without rebuilding or forking the SDK.

usage

// register before pipelines are loaded (e.g. in init() or early in main)
svc.RegisterExternalConfigurable("mycfg", func(lc logger.LoggingClient, sp bootstrapInterfaces.SecretProvider) interface{} {
    return &mycfg.MyConfigurable{lc: lc, sp: sp}
})

// MyConfigurable must expose methods like:
func (m *MyConfigurable) FilterByDeviceName(params map[string]string) interfaces.AppFunction {
    // build and return an AppFunction using params
}

changes

• add ConfigurableFactory and Service.Register/Unregister APIs
• build configurables list from builtin + registered factories
• iterate configurables when resolving pipeline functions

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/edgexfoundry/app-functions-sdk-go/blob/main/.github/CONTRIBUTING.md

PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)
  

Testing Instructions

New Dependency Instructions (If applicable)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.17391% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.34%. Comparing base (5dd841c) to head (7dd0900).

Files with missing lines	Patch %	Lines
internal/app/service.go	46.15%	6 Missing and 1 partial ⚠️
pkg/interfaces/mocks/ApplicationService.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1749      +/-   ##
==========================================
- Coverage   48.37%   48.34%   -0.04%     
==========================================
  Files          64       64              
  Lines        4934     4952      +18     
==========================================
+ Hits         2387     2394       +7     
- Misses       2337     2347      +10     
- Partials      210      211       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.