fix(tsc): audience property is not mandatory on verifyIdToken

[sergioregueira]

As code says, audience property is not mandatory on OAuth2Client.prototype.verifyIdToken() call despite the TypeScript interface currently requires it, so I have just updated the interface to make it optional.

[codecov]

Codecov Report

Merging #972 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #972   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files          19       19           
  Lines        4023     4023           
  Branches      488      454   -34     
=======================================
  Hits         3737     3737           
  Misses        286      286           

Impacted Files	Coverage Δ
src/auth/oauth2client.ts	96.30% <100.00%> (ø)

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[JustinBeckwith]

@bshaffer can I trouble you to take a look here?

[bcoe]

will wait on @bshaffer for feedback, but this looks reasonable to me.

[bshaffer]

I'm a bit confused as to what this actually does, as our verify-iap sample currently works with a null audience. But otherwise, this is LGTM

[sergioregueira]

This is just a fix of TypeScript definition, not business logic involved.

The sample you mention works because expectedAudience is defined as any (no explicit type on definition), and any matches any argument type (including string | string[]).