feat: add support for post requests for SSE

[Ancient-Dragon]

Pull request checklist

• Please read our contributor guide
• Consider creating a discussion on the discussion forum
  first
• Make sure the PR doesn't introduce backward compatibility issues
• Make sure to have sufficient test cases - hopefully because this is just changing the request handler I am ok. Let me know if another test is needed although I may need some help. (I tested locally using the graphql-sse library and can provide proof if needed.

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Changes in this PR

Change existing sse subscriptions to be a get request handler and move common code into a new method then add a post request handler which calls the same code.

Describe the new behavior from this PR, and why it's needed
Issue #236 - the ask was to add documentation but it looks like the feature isn't properly supported. I've added support for post requests (which will probably be the most common usecase)

Alternatives considered

Describe alternative implementation you have considered

[Ancient-Dragon]

Result using this in altair (with subscriptions using sse):

[berngp]

Thank you for the pr!

[Ancient-Dragon]

Thanks for approving, when do you think this'll be merged and released? I was hoping to use in our project fairly soon.

[berngp]

@Ancient-Dragon next few days.

[Ancient-Dragon]

Great many thanks!

[berngp]

Release v5.0.5 should be out soon.

[srinivasankavitha]

@Ancient-Dragon - thanks very much for your contribution. Would you also be able to update the documentation if you have time? It would greatly help us. If not, no worries, we will take care of it.

[Ancient-Dragon]

Hi @srinivasankavitha I have created the PR here: Netflix/dgs#100