feat: add support for self-signed certificates

First of all: very cool project! I think this intermediate layer between plugin and llm backend makes a lot of sense.

Now regarding this issue: There is currently no way (that I am aware of) to let llm-ls trust custom self-signed certificates. This makes it impossible to communicate securely with backends that use such certificates.

After looking into this a bit, it seems that this could be alleviated relatively easily by not using the `rustls-tls` feature when importing reqwest:
```
reqwest = { version = "0.11", default-features = false, features = ["json", "rustls-tls"] }
```

By default, reqwest will then use system-native TLS (see: https://docs.rs/reqwest/latest/reqwest/#tls), which would allow the user to simply trust self-signed certificates within the OS and llm-ls should then also automatically trust this certificate.