[ENH] Check that BIDS folders have a `dataset_description.json` file

[NicolasGensollen]

As per the BIDS specifications, the dataset_description.json file is mandatory.

This PR proposes the following:

• add a check for existence of a dataset_description.json file in the utility function check_bids_folder().
• small refactoring of this function (each check is moved to a separate function).
• add a DatasetType enumeration instead of relying on "BIDS" and "CAPS" strings (might move it later to a better place in the code base...).
• add unit tests for the added check and refactor the test function for check_bids_folder() to have smaller and more readable tests.

Note: An unused function _check_dataset_description_exists_in_caps() has been added to prepare for #1101

Requires data PR https://github.com/aramis-lab/clinica_data_ci/pull/54, https://github.com/aramis-lab/clinica_data_ci/pull/55, and https://github.com/aramis-lab/clinica_data_ci/pull/56 since some BIDS folders from the CI data do not have a dataset_description.json file.