Groundwork to prepare for arrays

[amunra]

We will soon be introducing an nd-array type in QuestDB and in the ILP protocol.

• The new array fields will be sent as binary.
• The existing Buffer is internally based on a mutable String type in Rust: This needs to be modified to internally use a Vec<u8> instead.
• The existing methods that expose the buffer's accumulated string also need to be removed and replaced with equivalent methods that expose bytes. C, C++ and Python APIs need to be adjusted accordingly.
• We also need to update our ureq dependency to 3.x. This will mean reworking some of the TLS logic since the new library more clearly decouples these details.

The resulting PR will thus effectively introduce a minor but nevertheless breaking change in the API.