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>

site/src/api/typesGenerated.ts

@@ -307,6 +307,11 @@ export const BypassRatelimitHeader = "X-Coder-Bypass-Ratelimit";
 // From codersdk/client.go
 export const CLITelemetryHeader = "Coder-CLI-Telemetry";
 
+// From codersdk/cors_behavior.go
+export type CORSBehavior = "passthru" | "simple";
+
+export const CORSBehaviors: CORSBehavior[] = ["passthru", "simple"];
+
 // From codersdk/workspacebuilds.go
 export interface CancelWorkspaceBuildParams {
 	readonly expect_status?: CancelWorkspaceBuildStatus;
@@ -492,6 +497,7 @@ export interface CreateTemplateRequest {
 	readonly require_active_version: boolean;
 	readonly max_port_share_level: WorkspaceAgentPortShareLevel | null;
 	readonly template_use_classic_parameter_flow?: boolean;
+	readonly cors_behavior: CORSBehavior | null;
 }
 
 // From codersdk/templateversions.go
@@ -2816,6 +2822,7 @@ export interface Template {
 	readonly time_til_dormant_autodelete_ms: number;
 	readonly require_active_version: boolean;
 	readonly max_port_share_level: WorkspaceAgentPortShareLevel;
+	readonly cors_behavior: CORSBehavior;
 	readonly use_classic_parameter_flow: boolean;
 }
 
@@ -3188,6 +3195,7 @@ export interface UpdateTemplateMeta {
 	readonly deprecation_message?: string;
 	readonly disable_everyone_group_access: boolean;
 	readonly max_port_share_level?: WorkspaceAgentPortShareLevel;
+	readonly cors_behavior?: CORSBehavior;
 	readonly use_classic_parameter_flow?: boolean;
 }