Add libsql-context-server extension

[nicholasq]

Added libsql-context-server as a submodule. Updated extensions.toml and .gitmodules accordingly.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Nicholas Quezada.
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[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @nicholasq on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[nicholasq]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[maxdeviant]

Thank you!