[oauth2client] Fixes mapping of FederatedSignonCertsResponse in verifyIdTokenAsync

[dropdevcoding]

Calling verifyIdTokencurrently causes the following error:

Error: No pem found for envelope: Error: No pem found for envelope: {"alg":"RS256","kid":"0a50b51f9357c0045baa6a7c8cafbfa1604c0edb"}
at OAuth2Client.verifySignedJwtWithCerts (/root/node_modules/google-auth-library/src/auth/oauth2client.ts:670:5)
at OAuth2Client.<anonymous> (/root/node_modules/google-auth-library/src/auth/oauth2client.ts:563:24)
at OAuth2Client.<anonymous> (/root/node_modules/google-auth-library/src/auth/oauth2client.ts:563:24)
at step (/root/node_modules/google-auth-library/build/src/auth/oauth2client.js:57:23)
at Object.next (/root/node_modules/google-auth-library/build/src/auth/oauth2client.js:38:53)
at Object.next (/root/node_modules/google-auth-library/build/src/auth/oauth2client.js:38:53)
at fulfilled (/root/node_modules/google-auth-library/build/src/auth/oauth2client.js:29:58)
at <anonymous>
at <anonymous>
at process._tickDomainCallback (internal/process/next_tick.js:228:7)

since the response of getFederatedSignonCertsAsync is not properly mapped in verifyIdTokenAsync which makes the cert lookup fail in verifySignedJwtWithCerts

This small PR fixes the issue.

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[dropdevcoding]

I signed it!

[googlebot]

CLAs look good, thanks!

[JustinBeckwith]

Great catch. Thanks for the fix!

[dropdevcoding]

Welcome anytime!