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

docs/modules/language-reference/pages/index.adoc

@@ -5059,8 +5059,6 @@ in the context of that module.
 [[glob-patterns]]
 === Glob Patterns
 
-Resources and modules may be imported at the same time by globbing with the <<globbed-imports>> and <<globbed-reads>> features.
-
 Pkl's glob patterns mostly follow the rules described by link:{uri-glob-7}[glob(7)], with the following differences:
 
 * `*` includes names that start with a dot (`.`).