Introduce extensible configuration fetcher SPI for JCasC

[somiljain2006]

Fixes #2266

Introduce a pluggable configuration fetcher API that allows Configuration as Code to load YAML from extensible sources while preserving existing behavior. Add default fetchers for local file system paths and HTTP/HTTPS URLs, along with a credential resolution abstraction supporting Jenkins credentials and bootstrap environment variables. Refactor configuration loading to use the new fetcher pipeline with managed resource lifecycles, and add unit and integration tests covering fetchers, credential providers, resource cleanup, and end-to-end configuration loading.

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