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/bindings-specification/pages/message-passing-api.adoc

@@ -210,6 +210,11 @@ class Http {
   ///
   /// Each rewrite must start with `http://` or `https://`, and must end with `/`.
   rewrites: Mapping<String, String>?
+
+  /// HTTP headers.
+  /// Each entry key is a glob pattern that is matched against outbound request URLs.
+  /// Each value is a map of headers that is added to the matching request.
+  headers: Mapping<String, Mapping<String, Listing<String>>>?
 }
 
 /// Settings that control how Pkl talks to HTTP proxies.

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 (`.`).

docs/modules/pkl-cli/partials/cli-common-options.adoc

@@ -158,6 +158,23 @@ This option is commonly used to enable package mirroring.
 The above example will rewrite URL `\https://pkg.pkl-lang.org/pkl-k8s/k8s@1.0.0` to `\https://my.internal.mirror/pkl-k8s/k8s@1.0.0`.
 ====
 
+.--http-header
+[%collapsible]
+====
+Default: (none) +
+Example: `**=User-Agent: My User Agent` +
+
+Additional headers to add to outbound HTTP requests.
+
+The syntax is `<glob pattern>=<header name>:<header value>`, and any whitespace after the leading colon is trimmed.
+
+The glob pattern is used to match against the URLs of outbound HTTP calls, and the value is the header name and value to add.
+In the case of multiple matches, every header is added.
+
+To describe a prefix match, the `\\**` wildcard should be added to the pattern.
+For example, `https?://example.com/**` matches against every request to host `example.com`.
+====
+
 .--trace-mode
 [%collapsible]
 ====

docs/modules/pkl-gradle/partials/gradle-common-properties.adoc

@@ -119,6 +119,19 @@ This option is commonly used to enable package mirroring.
 The above example will rewrite URL `\https://pkg.pkl-lang.org/pkl-k8s/k8s@1.0.0` to `\https://my.internal.mirror/pkl-k8s/k8s@1.0.0`.
 ====
 
+.httpHeaders: MapProperty<String, Map<String, List<String>>>
+[%collapsible]
+====
+Default: `null` +
+Example: `httpHeaders = ["**": ["User-Agent": ["My User Agent"]]]` +
+Additional headers to add to outbound HTTP requests.
+The key is a glob pattern that is used to match against the URLs of outbound HTTP calls, and the value is the header name and values to add.
+Each header value becomes its own HTTP header.
+
+To describe a prefix match, the `\\**` wildcard should be added to the pattern.
+For example, `https?://example.com/**` matches against every request to host `example.com`.
+====
+
 .powerAssertions: Property<Boolean>
 [%collapsible]
 ====