chore: add fastapi to client extra

[dennisrall]

Just a quickfix for #212.

It would be better not to have to install fastapi only for using the client, but in my opinion this is still better then getting errors when importing the RemoteRunnable after a pip install "langserve[client]".

Otherwise at least the readme should be updated.

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[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the username @dennisrall on file. In order for us to review and merge your code, please complete the Individual Contributor License Agreement here https://forms.gle/AQFbtkWRoHXUgipM6 .

This process is done manually on our side, so after signing the form one of the maintainers will add you to the contributors list.

For more details about why we have a CLA and other contribution guidelines please see: https://github.com/langchain-ai/langserve/blob/main/CONTRIBUTING.md.