Improve the typing of the Strategy class hierarchy.

[forty]

Description

The goal of this PR is to improve the typing of the Strategy class hierarchy.

The current expected ts error causes problems with some earlier version of TS.

[watch:ts] node_modules/passport-saml/lib/passport-saml/multiSamlStrategy.d.ts(10,5): error TS2416: Property 'generateServiceProviderMetadata' in type 'MultiSamlStrategy' is not assignable to the same property in base type 'Strategy'.
[watch:ts]   Type '(req: Request, decryptionCert: string, signingCert: string, callback: (err: Error, metadata?: string) => void) => void' is not assignable to type '(decryptionCert: string, signingCert?: string) => string'.

See #540 which seems to be affected by this too.

Also, it does not make any sense to have a method override a method with a different signature, just because they have the same name.

[cjbarth]

This looks good to me. I have a significant refactor of the type system in #548 which addresses some complex issues surrounding requiring a cert. I'm trying to get that landed before changes like this; though we'll certainly get this landed before we release v3. Feel free to have a look at it and offer feedback.

[cjbarth]

Is this something that you wanted fixed in the 2.x branch, or just in master?

[forty]

@cjbarth I'm actually mostly interested by the 2.x branch for now, since that's what I'm using at the moment. I'll open a PR for that branch too 👍

[cjbarth]

@forty Would you like to resolve the merge conflicts so we can get this landed as part of the upcoming 3.x release?

[forty]

Just rebased on master 👍

[cjbarth]

As we go along, I'm OK with adding types and making other minor changes in files we're touching to reduce the number of eslint errors.

[cjbarth]

@forty Would you like to get this rebased and we'll get it landed? If you need help, please let me know.

[forty]

@cjbarth I just rebased on latest master 👍

[cjbarth]

If you want this on 2.x, please create a PR there.