Unit tests and Jupyter notebooks use the latest release of OpenAssetIO
(see `examples/resources/requirements.txt` and `pyproject.toml`).

However, C++ tests, which currently must build OpenAssetIO from source,
were using the latest `main`.

So ensure we test against a consistent version of OpenAssetIO for both
C++ and Python tests by pinning the checkout of OpenAssetIO to the
latest release. Note that we cannot specify a semver range, so this
will get out of date on the next release.

Future CI work is required to design a more well-thought testing policy
e.g. testing against supported version ranges vs. `main` and nightlies.

Signed-off-by: David Feltell <[email protected]>

Part of OpenAssetIO/OpenAssetIO#1202. Add a Jupyter Notebook
illustrating the design and usage of the hybrid plugin system.

Bump the minimum versions of OpenAssetIO and BAL in `requirements.txt`
to support the required features for the notebook to run.

Use a custom hybrid plugin created especially for the notebook, rather
than attempting to sellotape existing managers together (e.g. BAL and
SimpleCppManager). This allows the notebook to avoid potentially
confusing readers with misleading text about configuring two completely
independent plugins to work together. For the custom plugin sources, use
linter config lifted from the main OpenAssetIO repo, with little
modification.

Signed-off-by: David Feltell <[email protected]>