Support insecure for OIDC integration

[Labiote]

Describe the feature request

The feature proposal aims to make it possible to deploy gatus in a development environment where we don't manage any trusted certificates. Thus, having an option to say whether or not we want to skip tls verification for OIDC provider. Something like this:

security:
  oidc:
    issuer-url: "https://example.okta.com"
    redirect-url: "https://status.example.com/authorization-code/callback"
    client-id: "123456789"
    client-secret: "abcdefghijk"
    scopes: ["openid"]
    insecure: true #<--- We skip certificate verification and don't face issue zith certificates signed by unknown authority or no certificate provided at all for the OIDC provider

I think this feature could improve development process for when working with Gatus in development/integration environments.

Why do you personally want this feature to be implemented?

When working in development environments, when using certificates trusted by unknown authority or no certificate at all, for our OIDC provider, we face the issue where gatus cannot reach our OIDC provider in these dev environments with no certificate managenment :

2025/08/26 12:46:44 [config.validateEndpointsConfig] Validated 5 endpoints
2025/08/26 12:46:44 [config.validateEndpointsConfig] Validated 0 external endpoints
2025/08/26 12:46:44 [store.Initialize] Creating storage provider of type=memory
panic: Get "https://our_oidc_provider_domain.com/realms/our_realm/.well-known/openid-configuration": tls: failed to verify certificate: x509: certificate signed by unknown authority
goroutine 9 [running]:
github.com/TwiN/gatus/v5/api.(*API).createRouter(0xc000484358?, 0xc00060d9a0)
	/app/api/api.go:122 +0xe8e
github.com/TwiN/gatus/v5/api.New(0xc00060d9a0)
	/app/api/api.go:39 +0x199
github.com/TwiN/gatus/v5/controller.Handle(0xc00060d9a0)
	/app/controller/controller.go:19 +0x1c
created by main.start in goroutine 1
	/app/main.go:51 +0x56

How long have you been using this project?

Several months

Additional information

No response