Add terragrunt cfg for OmniGCP

[AlCutter]

This PR adds:

• A cloud build trigger for building the OmniGCP docker image from the witness repo.
• A witness terraform module and terragrunt configs for spinning it up in dev.

Towards transparency-dev/witness#386.

[codecov-commenter]

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[mhutchinson]

We have code to generate the public key from the private key, and you referenced this in some docs the other day. Did something change that means the public key is now needed here?

[AlCutter]

You're right, and I think that's almost always going to be the right answer for recovering the public key in a code setting - and indeed in the first cut of this I dide just throw that line away.

However, I was then thinking about what happens next once someone's spun this up: they need to share the URL and identity with the world, at which point it's less of a technical issue and more just "convenience"; we already have the public key right here, so let's just print it out for the operator to just cut'n'paste. Then I figured that it's just as easy to pop it into somewhere durable so there's no stress if they mess up/forget to do the copy.

The alternative would be to have the omniwitness log the public key into its debug logging, which is fine but then we're asking the opr to go scrobbling in the log vs just storing it in a convenient location.