fix(auth): add missing name field to DecodedIdToken

[7hokerz]

The DecodedIdToken returned from the verifySessionCookie or verifyIdToken methods is guaranteed to have an optional name field at runtime, but the field is not present in the type declaration.
Therefore, the name field is recognized as any, requiring unnecessary type guards or casting. This is a PR to resolve this issue.

firebase docs link
In this document, the token has an optional name field.

Changes

• Add name?: string to DecodedIdToken
• Add a doc comment for the new field in src/auth/token-verifier.ts
• Update the generated API surface in etc/firebase-admin.auth.api.md

[gemini-code-assist]

Code Review

This pull request adds an optional name property to the DecodedIdToken interface in both the API definition and the token verifier source code to represent the user's display name. I have no feedback to provide.

[7hokerz]

@lahirumaramba @jonathanedey , could you please take a look when you have a moment?