feat: Generic header handler (#26)

* Added a builder method to add arbitrary headers one by one.

* Updated README with examples of auth() and insert_header() and section leading text about query building.

README.md

@@ -32,6 +32,77 @@ let body = resp
     .await?;
 ```
 
+Simple example with JWT auth
+
+```rust
+use postgrest::Postgrest;
+
+let client = Postgrest::new("https://your.postgrest.endpoint");
+let resp = client
+    .from("your_table")
+    .auth("VerySensitiveJWTValueAsStringOrStr")
+    .select("*")
+    .execute()
+    .await?;
+let body = resp
+    .text()
+    .await?;
+```
+
+Simplified example using an authenticated API gateway and JWT authorization (like SupaBase).
+
+```rust
+use postgrest::Postgrest;
+
+static HEADER_KEY: &str = "apikey";
+let header_value: String = "ExampleAPIKeyValue".to_string(); // EXAMPLE ONLY! 
+// Don't actually hard code this value, that's really bad. Use environment
+// variables like with the dotenv(https://crates.io/crates/dotenv) crate to inject
+
+let client = Postgrest::new("https://your.supabase.endpoint/rest/v1/");
+let resp = client
+    .from("your_table")
+    .auth(header_value)
+    .insert_header(HEADER_KEY, header_value)
+    .select("*")
+    .execute()
+    .await?;
+let body = resp
+    .text()
+    .await?;
+
+```
+
+**Secure** example with authenticated API gateway and JWT authorization using the dotenv crate to correctly retrieve sensitive values.
+
+```rust
+use postgrest::Postgrest;
+use dotenv;
+
+dotenv().ok(); 
+
+static HEADER_KEY: &str = "apikey";
+
+let client = Postgrest::new("https://your.supabase.endpoint/rest/v1/");
+let resp = client
+    .from("your_table")
+    .auth(env::var("supabase_public_api_key").unwrap().to_string()))
+    .insert_header(
+        HEADER_KEY, 
+        env::var("supabase_public_api_key").unwrap().to_string())
+    .select("*")
+    .execute()
+    .await?;
+let body = resp
+    .text()
+    .await?;
+
+```
+
+### Building Queries
+
+These examples assume you've already initialized the client.  The methods `.from()` and `.rpc()` initalizes the query builder inside the client.
+
 Using filters:
 
 ```rust

src/builder.rs

@@ -59,6 +59,31 @@ impl Builder {
         self
     }
 
+    /// Add arbitrary headers to the request.  For instance when you may want to connect
+    /// through an api gateway that needs an API key header in addition to the JWT.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use postgrest::Postgrest;
+    ///
+    /// static header_name: &'static str =  "foo";
+    /// let header_value: String = "bar".to_string();
+    ///
+    /// let client = Postgrest::new("https://your.postgrest.endpoint/rest/v1/");
+    /// client
+    ///     .from("table")
+    ///     .insert_header(header_name, header_value);
+    /// ```
+    pub fn insert_header(mut self, header_name: &'static str, header_value: String) -> Self {
+        self.headers.insert(
+            header_name,
+            HeaderValue::from_str(&header_value)
+                .expect("Couldn't convert supplied header value from String to str."),
+        );
+        self
+    }
+
     /// Performs horizontal filtering with SELECT.
     ///
     /// # Note
@@ -469,6 +494,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn with_insert_header() {
+        static A_HEADER_KEY: &str = "foo";
+        let a_header_value: String = "bar".to_string();
+        let builder = Builder::new(TABLE_URL, None).insert_header(A_HEADER_KEY, a_header_value);
+        assert_eq!(
+            builder.headers.get("foo").unwrap(),
+            HeaderValue::from_static("bar")
+        );
+    }
+
     #[test]
     fn select_assert_query() {
         let builder = Builder::new(TABLE_URL, None).select("some_table");