Additional functions to add

[MrPowers]

These are functions that are not implemented in Spark, but commonly requested. They should be implemented as Catalyst Expressions so they're performant for the community:

Datetime

• [X] beginningOfDay
• [X] beginningOfMonth
• [ ] beginningOfQuarter
• [ ] beginningOfWeek
• [ ] beginningOfYear
• [ ] endOfDay
• [ ] endOfMonth
• [ ] endOfQuarter
• [ ] endOfWeek
• [ ] endOfYear
• [ ] today
• [ ] tomorrow