This commit introduces new configuration options for BigQuery load jobs and external table definitions, aligning with recent updates to the underlying protos.

New options added:

- `time_zone`: Specifies the default timezone for parsing timestamps.
  (Applies to LoadJobConfig, ExternalConfig; CSV & JSON)
- `date_format`: Specifies the format for parsing DATE values.
  (Applies to LoadJobConfig, ExternalConfig; CSV & JSON)
- `datetime_format`: Specifies the format for parsing DATETIME values.
  (Applies to LoadJobConfig, ExternalConfig; CSV & JSON)
- `time_format`: Specifies the format for parsing TIME values.
  (Applies to LoadJobConfig, ExternalConfig; CSV & JSON)
- `timestamp_format`: Specifies the format for parsing TIMESTAMP values.
  (Applies to LoadJobConfig, ExternalConfig; CSV & JSON)
- `null_markers`: A list of strings to be interpreted as NULL.
  (Applies to LoadJobConfig, ExternalConfig (via CSVOptions); CSV only)
- `source_column_name_match_option`: Controls how source columns are matched to the schema.
  (Applies to LoadJobConfig, ExternalConfig (via CSVOptions); CSV only)

Changes include:
- Added corresponding properties (getters/setters) to `LoadJobConfig`, `LoadJob`, `ExternalConfig`, and `CSVOptions`.
- Updated docstrings and type hints for all new attributes.
- Updated unit tests to cover the new options, ensuring they are correctly handled during object initialization, serialization to API representation, and deserialization from API responses.

Adds miss, hit, and setter tests to `tests/unit/job/test_load_config.py`
for the following properties of `LoadJobConfig`:
- time_zone
- date_format
- datetime_format
- time_format
- timestamp_format
- null_markers
- source_column_name_match_option

These tests verify that the properties can be set, retrieved, and
correctly interact with the underlying configuration dictionary.

This commit restores user changes from commit a9b187f
that were inadvertently overwritten. These changes include:
- Addition of SourceColumnMatch enum in enums.py.
- Updates to docstrings in external_config.py and job/load.py.
- Renaming of source_column_name_match_option to source_column_match_strategy
  in LoadJobConfig and LoadJob, now using the SourceColumnMatch enum.
- Adjustments in related unit tests in test_load.py and test_external_config.py.

Additionally, this commit incorporates the new unit tests for all recently
added LoadJobConfig properties in tests/unit/job/test_load_config.py.

Corrections were made to tests/unit/job/test_load.py to align with the
renamed source_column_match_strategy property and its enum type.