Support XML format in S3 Source

## Tell us about the problem you're trying to solve
Currently S3 (via abstract files source) doesn't support syncing XML files.

## Describe the solution you’d like
Build a XmlParser [here](https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-s3/source_s3/source_files_abstract/fileformatparser.py) and add appropriate changes to [spec.py](https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-s3/source_s3/source_files_abstract/spec.py), [unit tests](https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-s3/unit_tests/test_fileformatparser.py) & documentation.

If those links are broken we may have refactored where that code sits and I forgot to update this, comment @ me and I'll fix them!

## Additional context
Adding this will support syncing xml files in any new files storage source that builds from [abstract-files-source](https://github.com/airbytehq/airbyte/tree/master/airbyte-integrations/connectors/source-s3/source_s3/source_files_abstract).