Support for datetime toStartOf functions

[AndrewSpeed]

Hi, we're making use of this library at Cloudsmith and in doing so have implemented a few of the clickhouse toStartOf* functions ourselves since django-clickhouse-backend doesn't have support for these yet

I'm more than happy to take on the work to add support to the library, and just wanted to get feedback on whether the maintainers are happy with these being added before starting work on the PR

Thanks for all your work creating and maintaining the library so far, it's been great for us to work with

[jayvynl]

Yes, I am very happy.