feat: allow bypassing current CORS magic based on template config (#18706)

Solves https://github.com/coder/coder/issues/15096

This is a slight rework/refactor of the earlier PRs from @dannykopping
and @Emyrk:
- https://github.com/coder/coder/pull/15669
- https://github.com/coder/coder/pull/15684
- https://github.com/coder/coder/pull/17596

Rather than having a per-app CORS behaviour setting and additionally a
template level setting for ports, this PR adds a single template level
CORS behaviour setting that is then used by all apps/ports for
workspaces created from that template.

The main changes are in `proxy.go` and `request.go` to:
a) get the CORS behaviour setting from the template
b) have `HandleSubdomain` bypass the CORS middleware handler if the
selected behaviour is `passthru`
c) in `proxyWorkspaceApp`, do not modify the response if the selected
behaviour is `passthru`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added support for configuring CORS behavior ("simple" or "passthru")
at the template level for all shared ports.
* Introduced a new "CORS Behavior" setting in the template creation and
settings forms.
* API endpoints and responses now include the optional `cors_behavior`
property for templates.
* Workspace apps and proxy now honor the specified CORS behavior,
enabling conditional CORS middleware application.
* Enhanced workspace app tests with comprehensive scenarios covering
CORS behaviors and authentication states.

* **Bug Fixes**
  * None.

* **Documentation**
* Updated API and admin documentation to describe the new
`cors_behavior` property and its usage.
* Added examples and schema references for CORS behavior in relevant API
docs.

* **Tests**
* Extended automated tests to cover different CORS behavior scenarios
for templates and workspace apps.

* **Chores**
* Updated audit logging to track changes to the `cors_behavior` field on
templates.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/database/dump.sql

@@ -73,6 +73,11 @@ CREATE TYPE connection_type AS ENUM (
     'port_forwarding'
 );
 
+CREATE TYPE cors_behavior AS ENUM (
+    'simple',
+    'passthru'
+);
+
 CREATE TYPE crypto_key_feature AS ENUM (
     'workspace_apps_token',
     'workspace_apps_api_key',
@@ -1750,7 +1755,8 @@ CREATE TABLE templates (
     deprecated text DEFAULT ''::text NOT NULL,
     activity_bump bigint DEFAULT '3600000000000'::bigint NOT NULL,
     max_port_sharing_level app_sharing_level DEFAULT 'owner'::app_sharing_level NOT NULL,
-    use_classic_parameter_flow boolean DEFAULT false NOT NULL
+    use_classic_parameter_flow boolean DEFAULT false NOT NULL,
+    cors_behavior cors_behavior DEFAULT 'simple'::cors_behavior NOT NULL
 );
 
 COMMENT ON COLUMN templates.default_ttl IS 'The default duration for autostop for workspaces created from this template.';
@@ -1803,6 +1809,7 @@ CREATE VIEW template_with_names AS
     templates.activity_bump,
     templates.max_port_sharing_level,
     templates.use_classic_parameter_flow,
+    templates.cors_behavior,
     COALESCE(visible_users.avatar_url, ''::text) AS created_by_avatar_url,
     COALESCE(visible_users.username, ''::text) AS created_by_username,
     COALESCE(visible_users.name, ''::text) AS created_by_name,