Add optional_mode and optional_fields support to backend service log_config

[db-vtx]

Is your feature request related to a problem? Please describe.
The log_config block for google_compute_backend_service in this module only exposes enable and sample_rate:

log_config = object({
  enable      = optional(bool)
  sample_rate = optional(number)
})

The underlying google_compute_backend_service resource's log_config also supports optional_mode and optional_fields, which control which optional fields (e.g. request/response headers) are included in load balancer logs. There's currently no way to set these through this module.

Describe the solution you'd like
Extend the log_config object type in variables.tf to include:

log_config = object({
  enable          = optional(bool)
  sample_rate     = optional(number)
  optional_mode   = optional(string)
  optional_fields = optional(list(string))
})

and pass them through in the dynamic "log_config" block in main.tf.

Describe alternatives you've considered
None — this is a straightforward passthrough of an existing provider field.

Additional context
Provider docs:

• https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_backend_service#optional_mode-1
• https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_backend_service#optional_fields-1