Rename the confusing endpoint `content_get` to `content_get_data`. This also
now works on one content as it is how it is used today.
Impacts:
- D3729 loader-core
- D3730 vault
- D3732 web
```
grep -r "content_get(" $SWH_ENVIRONMENT_HOME/**/swh/* | grep -v swh-storage
/home/tony/work/inria/repo/swh/swh-environment/swh-indexer/swh/indexer/tests/utils.py: def test_generate_content_get(self):
/home/tony/work/inria/repo/swh/swh-environment/swh-loader-core/swh/loader/package/npm/tests/test_npm.py: list(loader.storage.content_get(_expected_new_contents_first_visit))
/home/tony/work/inria/repo/swh/swh-environment/swh-vault/swh/vault/to_disk.py: contents_fetched = storage.content_get(contents_to_fetch)
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/common/service.py: c = _first_element(storage.content_get([content.sha1]))
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/common/service.py: content = _first_element(storage.content_get([content_sha1_bytes]))
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/browse/views/test_content.py: content_data = archive_data.content_get(content["sha1"])["data"]
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/browse/views/test_content.py: content_data = archive_data.content_get(content["sha1"])["data"]
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/browse/views/test_content.py: content_data = archive_data.content_get(content["sha1"])
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/api/views/test_content.py: expected_data = archive_data.content_get(content["sha1"])
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/api/views/test_content.py: expected_data = archive_data.content_get(content["sha1"])
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/data.py: cnt = next(storage.content_get([sha1]))
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/common/test_service.py: expected_data = archive_data.content_get(expected_dir_entry["checksums"]["sha1"])
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/common/test_service.py: expected_content = archive_data.content_get(content["sha1"])
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/conftest.py: def content_get(self, cnt_id):
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/conftest.py: cnt = next(self.storage.content_get([cnt_id_bytes]))
/home/tony/work/inria/repo/swh/swh-environment/swh-web/swh/web/tests/strategies.py: get_tests_data()["storage"].content_get([hash_to_bytes(c["sha1"])])
```