Proposed PR: configurable time encoding

Hi, and thank you for an excellent library that I've used for years!

This driver stores time.Time values as strings: https://github.com/mattn/go-sqlite3/blob/85436841b33e86c07dce0fa2e88c31a97c96a22f/sqlite3.go#L219 but for compactness and consistency I always prefer to use unix timestamps. This has meant always modifying my Go code to save an int version of whatever time I'm using.

I'd like to propose a new driver configuration parameter, _time_format, that would let one opt into different storage formats. This lets you use time.Time directly but get the encoding you want. I've built a version that allows unix and unix_ms formats, and it is a small (~dozen LOC) change: kalafut/go-sqlite3.

Other formats could be added in the future too, which might be useful (e.g. I noticed https://github.com/mattn/go-sqlite3/issues/951#issuecomment-985078746). Note that this is completely opt-in and only affects the way values are written. Reading/parsing is unchanged.

Is this of any interest? If so I can prepare a proper PR with better, consolidated tests, and some docs.

Thanks for taking a look.

Start in sqlite3.go around the time.Time handling linked in the issue, then compare the proposed implementation in the referenced fork. Add consolidated tests for the unix and unix_ms formats and document the _time_format option, while preserving the existing reading behavior.