Azure Event Hubs client library samples with persistent checkpointing for TypeScript

Upgrade TypeScript compiler version to 4.1.2 (#12770)

* upgrade TS version and fix compilation issues

* upgrade the linting parser version and fix new linting issues

* fix cosmos sample

* address feedback

* fix linting issues in formrecognizer tests

* use unknown instead of any across our code

* address more issues

* cleanup package.json in core-http

* revert noisy linting changes caused by vanilla eslint rules not TS aware

* allow the poller to have results of type void

* fixing samples

* fix keyvault-certificates' sample