Improve HTTP headers logic (#1584)

* Relax forbidden headers constraints
  - remove restriction on browser-related headers
- allow any glob pattern (no need to end with `/` or `*`, because glob
patterns already require users to explicitly declare prefix matches if
that's the intention)
* Replace `List<Pair<, ...>>`; use `Map<String, ...>` instead
* Use glob pattern strings as an API throughout, instead of `Pattern`
(e.g. in `HttpClientBuilder`)
* Add HTTP headers to message passing API
* Add HTTP headers to executor API (introduces `ExecutorSpiOptions4`)
* Add tests for Gradle, CLI, and pkl-executor invocations
* Improve documentation
* Add `isGlobPattern` API to class `String` for in-language validation
of http headers
* Behavior change: make sure explicitly configured `User-Agent` in
`HttpClientBuilder` can be shadowed by headers (allows users to set
`--http-header "**=User-Agent: My User Agent"` and for this to be the
only user agent).

CC @kyokuping

stdlib/base.pkl

@@ -1323,6 +1323,13 @@ external class String extends Any {
   /// Tells if this string is a valid regular expression according to [Regex].
   external isRegex: Boolean
 
+  /// Tells if this string is a valid glob pattern.
+  ///
+  /// For reference, see
+  /// <https://pkl-lang.org/main/current/language-reference/index.html#glob-patterns>.
+  @Since { version = "0.32.0" }
+  external isGlobPattern: Boolean
+
   /// Tells if this is a valid base64-encoded string.
   ///
   /// Facts: