Add streaming option to FileLoader. (#51)

## Description
Add streaming option to FileLoader. Default behaviour is batch.

## Related Issue
[Issue 50
](#50)

## Motivation and Context
Having a `streaming` option for the `FileLoader` would be beneficial
during UT's or for people that are not on Databricks and can't make use
of the Databricks proprietary `Autoloader`.

## How Has This Been Tested?
- UT's
- On Databricks


## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

## Checklist:
<!--- Go over all the following points, and put an `x` in all the boxes
that apply. -->
<!--- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
- [x] My code follows the code style of this project.
- [ ] My change requires a change to the documentation.
- [ ] I have updated the documentation accordingly.
- [x] I have read the **CONTRIBUTING** document.
- [x] I have added tests to cover my changes.
- [ ] All new and existing tests passed.

src/koheesio/spark/readers/file_loader.py

@@ -97,6 +97,7 @@ class FileLoader(Reader, ExtraParamsMixin):
     schema_: Optional[Union[StructType, str]] = Field(
         default=None, description="Schema to use when reading the file", validate_default=False, alias="schema"
     )
+    streaming: Optional[bool] = Field(default=False, description="Whether to read the files as a Stream or not")
 
     @field_validator("path")
     def ensure_path_is_str(cls, v):
@@ -106,8 +107,9 @@ def ensure_path_is_str(cls, v):
         return v
 
     def execute(self):
-        """Reads the file using the specified format, schema, while applying any extra parameters."""
-        reader = self.spark.read.format(self.format)
+        """Reads the file, in batch or as a stream, using the specified format and schema, while applying any extra parameters."""
+        reader = self.spark.readStream if self.streaming else self.spark.read
+        reader = reader.format(self.format)
 
         if self.schema_:
             reader.schema(self.schema_)

tests/spark/readers/test_file_loader.py

@@ -1,5 +1,7 @@
 import pytest
 
+import pyspark.sql.types as T
+
 from koheesio.spark import AnalysisException
 from koheesio.spark.readers.file_loader import (
     AvroReader,
@@ -106,6 +108,15 @@ def test_json_reader(json_file):
     assert actual_data == expected_data
 
 
+def test_json_stream_reader(json_file):
+    schema = "string STRING, int INT, float FLOAT"
+    reader = JsonReader(path=json_file, schema=schema, streaming=True)
+    assert reader.path == json_file
+    df = reader.read()
+    assert df.isStreaming
+    assert df.schema == T._parse_datatype_string(schema)
+
+
 def test_parquet_reader(parquet_file):
     expected_data = [
         {"id": 0},