Handle custom decoders as part of setting fields in post_create

[modular-magician]

GoogleCloudPlatform/magic-modules#12939 started triggering generation of post-create code, but it missed adding support for decoders (which are already used by the equivalent code for pulling computed fields from operation responses.) I also added setting of fields for resources that use PollAsync on create.

Decoder support is necessary to properly handle some resources, like bigqueryanalyticshub_listing_subscription. In this specific case, we do end up just moving some code around from one custom code template to another; however, this may reduce the need for post_create custom code on other resources as well.

However, we can't run decoders for PollAsync resources (because the decoder returns nil until the resource exists).

(I think we should split this logic into a separate function & use it for post-create for OpAsync resources as well - they currently rely on "identity" fields - but I'm leaving that for a separate PR.)

I had to mark monitored project as using async for create to prevent it trying to decode the operation that it gets on Create.

Part of hashicorp/terraform-provider-google#22214

Fixed hashicorp/terraform-provider-google#22239

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Derived from GoogleCloudPlatform/magic-modules#13520