docs: update README

[cflee]

Proposing some updates to the README, based on my experience integrating against Singpass and Myinfo portions of it, and supporting others who were integrating against Corppass and sgID.

• Remove all references to some no-longer-in-use options that presumably date back to the SAML Singpass/Corppass era, i.e. SIGN_ASSERTION, SIGN_RESPONSE, ENCRYPT_ASSERTION and RESOLVE_ARTIFACT_REQUEST_SIGNED. This should fix users thinking these could be used to modify behaviour of the current Singpass/Corppass/Myinfo/sgID implementations.
• Reorganise the README to be per use case, and document most options on this per use case basis.
  • I propose that this is clearer as it allows users to read just what they need instead of seeing a central list of options and having to read some detailed conditions on which options apply to which use case.
  • While the env vars or headers are generally reused across use cases, they have subtly different behaviour, e.g. sgID doesn't support per-request no-login-page default profile override via custom headers unlike Singpass/Corppass, and Myinfo v3 and sgID have the client certificates configured differently (Myinfo v3 requires two separate env vars for some legacy reason, I think some no-longer-in-use library required a separate dedicated public key PEM file, while sgID just requires one env var).
  • Deprioritise the Singpass/Corppass v1 to the end.
• Add some hints, like sgID only working for the static Myinfo personas, being able to use any client_id, Myinfo is v3 only and not v4.
• Update a note at the top to note that Mockpass also supports sgID. This should fix sgID users thinking that they have to do an additional integration against Singpass to use Mockpass for integration/e2e testing purposes.
• Suggest an alternative of running directly with npx without npm install.

[LoneRifle]

lgtm