bfetch (#52888)

* feat: 🎸 add bfetch plugin stub

* feat: 🎸 add sample routes

* feat: 🎸 implement streaming response

* feat: 🎸 add Defer class

* refactor: 💡 move Defer inot /common folder

* feat: 🎸 add fromStreamingXhr() method

* feat: 🎸 add split method

* feat: 🎸 add fetchStreaming() function

* test: 💍 fix test after refactor

* test: 💍 add tests for fetStreaming() method

* refactor: 💡 move removeLeadingSlash() to /common folder

* feat: 🎸 expor stateful fetchStreaming() throuh plugin contract

* chore: 🤖 clean up bfetch

* chore: 🤖 prepare to replace ajax_stream by bfetch

* Change ajax_stream to use new-line delimited JSON

* refactor: 💡 move batched_fetch to use bfetch service

* refactor: 💡 make use of defer() utility from kibana_utils

* chore: 🤖 remove ajax_stream library

* fix: 🐛 fix tests and inject fetchStreaming() method as dep

* refactor: 💡 make split() operator more readable

* refactor: 💡 improvee PR according to feedback

* docs: ✏️ add fetchStreaming() reference

* refactor: 💡 use NP logger, rename to createNDJSONStream()

* chore: 🤖 adress Luke's review comments

* chore: 🤖 add missing type