feat: `bgp` block can now be optional

[tanguynicolas]

I have a customer who has not defined (from console) the bgp block. When I use the module, it creates a change. I don't currently have the option of setting the ASN to null to avoid creating the block.

The purpose of this PR is to support my use case. To reflect a little more the google_compute_router_nat resource which doesn't require the bgp block.

[tanguynicolas]

Sorry, bad commit message. It's a feature not a chore.
If you're ok with the changes I've made, let me know and I'll re-create the commit with the message feat: bgp block can now be optional

[imrannayer]

@tanguynicolas thx for the PR. can you plz fix lint issues. See contributing guide

[imrannayer]

/gcbrun

[imrannayer]

/gcbrun