feat: allow providing custom nonce

[vonovak]

Uses: openid/AppAuth-iOS#788. Motivation is explained there and also in issue #135

Fixes: #135

Supersedes: #244. It takes a slightly different approach where nonce is not provided via GIDConfiguration but via a parameter to signInWithPresentingViewController/Window

[google-cla]

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[mdmathias]

@vonovak did you sign the CLA? I can take a look at the PR after you confirm.

[vonovak]

hello @mdmathias yes I did! :)

[vonovak]

hello @mdmathias, would you please re-review the PR? Thank you! 🙂

[GNUGradyn]

Can we get this merged? This issue is blocking my app

[NorseGaud]

Bumpziez

[mdmathias]

@vonovak could you take a look at the test failures? An example:

- WARN | xcodebuild: /Users/runner/work/GoogleSignIn-iOS/GoogleSignIn-iOS/GoogleSignIn/Tests/Unit/OIDAuthorizationRequest+Testing.h:23:17: warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Wnullability-completeness]

[vonovak]

@mdmathias can you re-run the workflows? TY 🙂

[elyobo]

Tests all passing @mdmathias 👀

[vonovak]

@mdmathias may I ask if there's anything blocking this PR from merging? The discussion is resolved and CI is green.
Is there anything we're waiting for?
Thank you