feat: allow oauth in databricks provider

[salman1993]

i think the following will work with our generic DatabricksProvider, since all we need is the host to try to oauth:

/// Either token must be set or use_oauth must be true
pub struct DatabricksProviderConfig {
    pub host: String,
    pub token: Option<String>,
    pub use_oauth: bool,
}

order of checking:

1. config.token is used if its provided
2. check env DATABRICKS_TOKEN
3. if config.use_oauth -> oauth with host to get token

Open Questions

1. not sure if (2) and (3) should be swapped. we can also remove use_oauth depending on whether we wanna be explicit or not
2. should we expose other oauth config like client_id, redirect_url, cache_path
3. databricks_oauth.rs adds a bunch of dependencies so maybe we wanna make this an optional package that's only installed if you wanna use databricks?