Refactor DatasetRecord to use attrs

Why these changes are being introduced:
* Reworking the dataset partitions to use the [year, month, day]
of the 'run_date' means that parquet files for different 'source' runs
on the same 'run_date' get written to the same partition directory.
Therefore, it is crucial that the timdex_dataset_api.write method
retrieves the correct partition columns from the (batches) of DatasetRecord
objects. The DatasetRecord class has been refactored to adhere
to the following criteria:

1. When writing to the dataset, and therefore serializing DatasetRecord objects,
   year, month, day should be derived from the run_date and should not be modifiable
2. If possible, avoid parsing a datetime string 3 times for each partition column

How this addresses that need:
* Refactor DatasetRecord to use attrs
* Define custom strict_date_parse converter method for 'run_date' field
* Simplify serialization method to rely on converter for 'run_date'
  error handling
* Remove DatasetRecord.validate
* Include attrs as a dependency

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-432

Pipfile

@@ -4,6 +4,7 @@ verify_ssl = true
 name = "pypi"
 
 [packages]
+attrs = "*"
 boto3 = "*"
 duckdb = "*"
 pandas = "*"
@@ -14,15 +15,14 @@ black = "*"
 boto3-stubs = {version = "*", extras = ["s3"]}
 coveralls = "*"
 ipython = "*"
+moto = "*"
 mypy = "*"
+pandas-stubs = "*"
 pre-commit = "*"
+pytest-mock = "*"
 pyarrow-stubs = "*"
 pytest = "*"
 ruff = "*"
 setuptools = "*"
-pandas-stubs = "*"
-moto = "*"
-pytest-mock = "*"
-
 [requires]
 python_version = "3.12"