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>

codersdk/organizations.go

@@ -206,6 +206,9 @@ type CreateTemplateRequest struct {
 	// true, and is why `*bool` is used here. When dynamic parameters becomes
 	// the default, this will default to false.
 	UseClassicParameterFlow *bool `json:"template_use_classic_parameter_flow,omitempty"`
+
+	// CORSBehavior allows optionally specifying the CORS behavior for all shared ports.
+	CORSBehavior *CORSBehavior `json:"cors_behavior"`
 }
 
 // CreateWorkspaceRequest provides options for creating a new workspace.