Harden the build external components feature

[dbarker]

Discussed in the SIG meeting on 5/14/25

• The feature to build external components provided by opentelemetry-build-external-component.cmake supports environment variables and fetching from URLs now.
  • if the OPENTELEMETRY_EXTERNAL_COMPONENT_PATH or OPENTELEMETRY_EXTERNAL_COMPONENT_URL environment variables are set then no cmake changes are needed to include an external component in the build.
• We discussed removing the environment variables so the feature requires a cmake parameter OPENTELEMETRY_EXTERNAL_COMPONENT_PATH to be set in order to function.