`StreamWriter.start_tls` should allow overriding `server_side`

Feature or enhancement

Proposal:

start_tls() is used to upgrade a connection to a secure version.
Currently the server_side parameter is calculated based on the existence of a callback which is only set by start_server().

While for most cases the server should be performing the server role for the TLS handshake, there's a few instances the opposite is desired.

For my personal use case I've duplicated the logic and added a new kwarg to set it manually and it works.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Locate the StreamWriter.start_tls entry point and inspect how start_server supplies server_side. Define the smallest public-API change that allows an explicit override while preserving current behavior, add focused coverage for the client and server roles, and run the relevant asyncio tests.