LowercaseTextProcessor for text normalization

[suparshwa31]

This PR adds a new LowercaseTextProcessor to the genai_processors/contrib module. It converts incoming text parts to lowercase while preserving non-text parts and all metadata.

Changes Introduced

• Added lowercase_text_processor.py under genai_processors/contrib
• Implemented LowercaseTextProcessor inheriting from PartProcessor
• Preserves metadata and non-text parts
• Added lowercase_text_processor_test.py under gene_processors/tests

[ArchishmanSengupta]

hey @suparshwa31 looks like you have already raised the PR for my issue #11 but i would like to have the maintainers check this and see if it's at all valid for this library. Let's have a proper discussion and let's not write AI slop code for the sake of contribution :))

[ArchishmanSengupta]

Please keep in mind that the maintainers spend valuable time on every PR and issue, so let’s make the most of everyone’s time 😊