Removing getCertificate() from the Credential interface

[hiranya911]

Credential is a public interface and is declared as follows in the typings:

interface Credential {
    getAccessToken(): Promise<admin.GoogleOAuthAccessToken>;
}

But in the actual implementation (credential.ts), there's an additional method in the interface:

firebase-admin-node/src/auth/credential.ts

Lines 291 to 294 in 843df12

	export interface Credential {
	getAccessToken(): Promise<GoogleOAuthAccessToken>;
	getCertificate(): Certificate;
	}

This can cause issues if the developer implements the public Credential interface, and passes it through AppOptions. Internal code that expects to find a getCertificate() method will fail.

This PR addresses this issue by separating the getCertificate() method from the Credential interface.

Resolves #573

[samtstern]

Some other places:

firebase-admin-node/src/storage/storage.ts

Line 73 in 2b40313

const cert: Certificate = app.options.credential.getCertificate();

firebase-admin-node/src/utils/index.ts

Line 72 in 9f770b4

const cert: Certificate = options.credential.getCertificate();