[Vertex AI] Add Citation.publicationDate

[andrewheard]

Added publicationDate to Citation.

[andrewheard]

@paulb777 Note: Android is defaulting values to 1 if not specified (https://github.com/firebase/firebase-android-sdk/blob/2dd9beb5525a5bcadb436ab118cda11945d4a5d5/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt#L256-L265) prior to converting to their platform type for dates (Calendar) rather than throwing.

JS isn't doing any conversions: https://github.com/firebase/firebase-js-sdk/blob/9c645aa3589ab234b6b4055bb31a96511cdd5812/packages/vertexai/src/types/responses.ts#L179-L187

[andrewheard]

I updated the public API to use DateComponents instead of Date. Devs can call publicationDate?.date to get a Date but also have the ability to inspect individual properties like day for nil if they want to adjust how to format dates in UIs, etc.

[ncooke3]

@paulb777 Note: Android is defaulting values to 1 if not specified (https://github.com/firebase/firebase-android-sdk/blob/2dd9beb5525a5bcadb436ab118cda11945d4a5d5/firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt#L256-L265) prior to converting to their platform type for dates (Calendar) rather than throwing.

JS isn't doing any conversions: https://github.com/firebase/firebase-js-sdk/blob/9c645aa3589ab234b6b4055bb31a96511cdd5812/packages/vertexai/src/types/responses.ts#L179-L187

Note that excluding a field in a DateComponents instance will default to 1. Matching the behavior in Android.