[WIP] Add standard User-Agent header to SDK

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Send a standard User-Agent: sdk-name/version header</issue_title>
<issue_description> 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-flutter-sdk/<version>.
• When the version is not easily discoverable at runtime, default to flagsmith-flutter-sdk/unknown.

Reference work:

• feat: add User-Agent header with SDK version flagsmith-go-client#181
• feat: Send a standard User-Agent: sdk-name/version header flagsmith-java-client#187
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Send a standard User-Agent: sdk-name/version header #82

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