Split Splice source updater module

[danielporterda]

Summary

• Moves the existing Splice OpenAPI source updater into a source-specific module without adding a new source target.

Stack

This is PR 4/8 in the generated-reference automation split.

Base: generated-reference-03-dashboard-github-token
Head: generated-reference-04-source-updater-split

Validation

• python3 -m pytest tests/test_update_generated_reference_sources.py tests/test_generate_network_component_versions.py tests/test_wallet_kernel_nav.py tests/test_docs_env.py; python3 scripts/update_generated_reference_sources.py --help; python3 -m py_compile source updater files; git diff --check