[Runtime] Add support for Lightning source directory to be used in runtime build

[mlxd]

Context: This PR allows Lightning to be cloned and used in-place with Catalyst's runtime, allowing faster turnaround and development against both packages. In addition, it overcomes some issues with Python package issues observed at build time by updating the runtime pybind11 and requiring Python to be found before proceeding.<\del>

Description of the Change: Added LIGHTNING_SRC_PATH CMake argument to specify a full path to a local Lightning repository, and all source will be used in place for a runtime build.

Benefits: Enables faster turnaround for development between Lightning and Catalyst. Can allow a multi-version clone of each repo and compilation steps to be validated and tested.

Possible Drawbacks:

Related GitHub Issues:

[mlxd]

[sc-63110]

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[mlxd]

Exploring whether https://github.com/PennyLaneAI/pennylane-lightning/pull/725/ will fix Kokkos failures

UPDATE: No further LK issues.

[mlxd]

Happy to approve 💯 Would love to figure the pybind/findpython issue at some point though if you have time.

Sounds good. Yea, I'll do some delving once time permits.