Add schema utility functions

[jsheunis]

This PR:

• Updates required fields in schemas, specifically removing name from the list of required fields of the dataset schema and adding metadata_sources to the list of required fields of both the dataset and file schemas
• Updates tests based on the above change
• Closes https://github.com/datalad/datalad-catalog/issues/376
• Closes https://github.com/datalad/datalad-catalog/issues/377
• Adds the ability get the base structure of a valid metadata item (with minimum required fields or all expanded fields), based on the current schema (either in the package of an argument-provided catalog). This functionality is contained in the new schema_utils module.
• Closes https://github.com/datalad/datalad-catalog/issues/346

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (8be6ec8) 86.19% compared to head (ba656ce) 86.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #378 +/- ## ========================================== + Coverage 86.19% 86.22% +0.02% ========================================== Files 43 45 +2 Lines 2709 2794 +85 ========================================== + Hits 2335 2409 +74 - Misses 374 385 +11 ``` | [Flag](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | `79.31% <91.01%> (+0.24%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [datalad\_catalog/constants.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL2NvbnN0YW50cy5weQ==) | `100.00% <100.00%> (ø)` | | | [datalad\_catalog/tests/test\_add.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL3Rlc3RzL3Rlc3RfYWRkLnB5) | `100.00% <100.00%> (ø)` | | | [datalad\_catalog/tests/test\_schema\_utils.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL3Rlc3RzL3Rlc3Rfc2NoZW1hX3V0aWxzLnB5) | `100.00% <100.00%> (ø)` | | | [datalad\_catalog/translate.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL3RyYW5zbGF0ZS5weQ==) | `85.18% <ø> (ø)` | | | [datalad\_catalog/utils.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL3V0aWxzLnB5) | `80.31% <100.00%> (+0.64%)` | :arrow_up: | | [datalad\_catalog/schema\_utils.py](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9jYXRhbG9nL3NjaGVtYV91dGlscy5weQ==) | `87.87% <87.87%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/datalad/datalad-catalog/pull/378/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad)

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