Send a standard `User-Agent: sdk-name/version` header

[khvn26]

SDK to include an explicit User-Agent HTTP header on all outbound calls:

• Use the SDK’s package/version at runtime.
• Format strictly as flagsmith-python-sdk/<version>.
• When the version is not easily discoverable at runtime, default to flagsmith-python-sdk/unknown.

[khvn26]

Released as part of https://github.com/Flagsmith/flagsmith-python-client/releases/tag/v4.0.0.