feat: add support for custom headers in SSE transport

[04cfb1ed]

Description

Add support for custom headers in SSE transport, this allows communication with secured servers

closes #5152

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[sestinj]

@04cfb1ed instead of just the "headers" field, I think we should use the full requestOptions object that exists in index.d.ts (it's already used for models, context providers, and more)

[04cfb1ed]

The requested changes were done, but one test it's failing with ETIMEDOUT

Installing extensions...
Error while installing extensions: AggregateError [ETIMEDOUT]: 
AggregateError [ETIMEDOUT]: 
Error: Command failed: ELECTRON_RUN_AS_NODE=1 "/home/runner/work/continue/continue/extensions/vscode/e2e/storage/VSCode-linux-x64/code" "/home/runner/work/continue/continue/extensions/vscode/e2e/storage/VSCode-linux-x64/resources/app/out/cli.js" --force --install-extension "ms-vscode-remote.remote-wsl" --extensions-dir=/home/runner/work/continue/continue/extensions/vscode/e2e/.test-extensions

From here #4866 (comment), I understand that the error is not from the new code

(I have the same error in #5517 but in another test)

[sestinj]

@04cfb1ed it looks like the tests are passing here on GitHub, so I'm not concerned about this. However, if you were interested in further debugging the problem I'm sure this would be useful for others trying to run local tests on the same OS as you