chore: allow external Buffer flushability checking

[ischeinkman]

Added a Buffer::check_can_flush method for checking if a Buffer can be flushed with Sender::flush.

Usecase

In our application we're operating in an async setting with a custom async HTTP library & writer. At the moment we need to isolate the QuestDB sending logic into its own set of threads because it doesn't expose enough information to be able to flush the Buffer ourselves. However, by including this check_can_flush call we can now recreate the entirety of Sender::flush_impl using our custom library and async system.

[CLAassistant]

All committers have signed the CLA.

[amunra]

Fair :-)

What HTTP library are you using, out of curiosity?

[amunra]

I've merged this PR, but note that we're adding array support soon which will also introduce new minor breaking changes to the API. Some of these have already been introduced in the main branch.

As such I'll delay releasing version 5.0.0 until those changes are ready.
For now, you should be able to use the questdb-rs dependency tied to the specific commit:

[dependencies]
questdb-rs = { git = "https://github.com/questdb/c-questdb-client", rev = "5408aa05ea9a5f16cbadc4f6c585cd6f8e8034d3" }