Field method refactor for Springshare transform

Why these changes are being introduced:
* These updates are required to implement the architecture described
in the following ADR: https://github.com/MITLibraries/transmogrifier/blob/main/docs/adrs/0005-field-methods.md

How this addresses that need:
* Refactor transform class SpringshareOaiDc to contain get_*
methods for optional fields
* Update tests for SpringshareOaiDc transform

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-283

tests/conftest.py

@@ -111,6 +111,40 @@ def oai_pmh_records():
     return XMLTransformer.parse_source_file("tests/fixtures/oai_pmh_records.xml")
 
 
+# springshare - libguides ##########################
+@pytest.fixture
+def springshare_libguides_record_optional_fields_blank():
+    source_records = OaiDc.parse_source_file(
+        "tests/fixtures/oai_dc/springshare/libguides/libguides_record_optional_fields_blank.xml"
+    )
+    return next(source_records)
+
+
+@pytest.fixture
+def springshare_libguides_record_optional_fields_missing():
+    source_records = OaiDc.parse_source_file(
+        "tests/fixtures/oai_dc/springshare/libguides/libguides_record_optional_fields_missing.xml"
+    )
+    return next(source_records)
+
+
+# springshare - research databases ##########################
+@pytest.fixture
+def springshare_research_databases_record_optional_fields_blank():
+    source_records = OaiDc.parse_source_file(
+        "tests/fixtures/oai_dc/springshare/research_databases/research_databases_record_optional_fields_blank.xml"
+    )
+    return next(source_records)
+
+
+@pytest.fixture
+def springshare_research_databases_record_optional_fields_missing():
+    source_records = OaiDc.parse_source_file(
+        "tests/fixtures/oai_dc/springshare/research_databases/research_databases_record_optional_fields_missing.xml"
+    )
+    return next(source_records)
+
+
 @pytest.fixture
 def timdex_record_required_fields():
     return timdex.TimdexRecord(