Added support for server sent events in Krakend Community Edition

[Harsh-Naicker]

• Added support for server sent events so that they are not buffered in Krakend
• Added testing code to validate new implementation.

Related issue: #924

[alombarte]

Hello @Harsh-Naicker , thank you for submitting an interesting feature to KrakenD. A few quick comments (without deeply reviewing the code). These are more form than content:

1. The SSE feature shouldn’t live in KrakenD-CE since that repo is for dependency stacking only, it needs its own repo.
2. Using SSE must enforce no-op output encoding programmatically, not rely on config.
3. All tests must be in Go, not JS; currently, the test setup is out of scope for KrakenD’s core testing practices.

[Harsh-Naicker]

@alombarte Thank you so much for taking a brief look at my PR. Should I do the following:

1. Wait for your detailed review on the code.
2. Create a new repository with the SSE support code with all the comments addressed from your review.
3. Update this PR to use the new module as a dependency.

Awaiting your kind response.