DPLT-1044 Expose fetch to VM runner

[morgsmccauley]

Now that we have the ability to write custom metrics from within the runner, we can start writing the 'lag behind social'. But to do that we need to fetch the latest post block height from api.social.near. For that we need access fetch.

[gabehamilton]

Can we wrap this in a context method? Adding fetch gives developers lots of functionality (webhook). That is functionality we want to provide but likely will want to wrap in various ways over time: allowlist domains perhaps, check for SSRF attacks.

With webhooks, instead of Indexers developers can build Reactors. We may want to run these differently or provide different options. That starts with knowing which animal we're dealing with.