[PLUGIN-1771] Add Streaming support for excel source

[psainics]

Add Streaming support for excel source

Jira : PLUGIN-1771

Description

Excel plugin consumes huge memory when reading large files.
This is due to it loading the complete file in it's memory.

This PR adds streaming support xlxs filetype.

• As we won't be able to stream old xls binary based format we use magic bytes to detect the filetype.

UI Field

• No Changes made to widget json.

Docs

• No Changes made to docs.

Code change

• Modified ExcelInputFormat.java

Unit Tests

• No Changes made to unit tests.

[fernst]

LGTM, can we add some tests for file reads? Just create some dummy xls/xlsx files, read and ensure output matches input.

[psainics]

This has been tested by QA!
We need to release in this CDF version so we are merging this PR as is, will raise a new PR adding test for reading dummy files !