Event payload modification using CEL

[dibyom]

From @bigkevmcd in #49 (comment)

Now that CEL interceptor is in there, I'd like to see if we can't agree on how to do CEL munging, i.e. the original values implementation in early days of that branch.

triggers:
- name: cel-trig
interceptor:
cel:
filter: "headers.match('X-GitHub-Event', 'pull_request')"
values:
"pr.url": body.pull_request.url
"pr.short_sha": truncate(body.head.sha, 7)
I'd propose something similar, I don't have a better name than values, but I think it should be a list of key/values, with "key" and "expression" as the keys for each thing to update.

so...

triggers:

• name: cel-trig
  interceptor:
  cel:
  filter: "headers.match('X-GitHub-Event', 'pull_request')"
  values:
  - key: pr.url
  expression: body.pull_request.url
  - key: pr.short_sha
  expression: truncate(body.head.sha, 7)
  This would be used in the same way as the original values implementation, i.e. if the filter was successful, using sjson to set the keys in the body to the values returned by the expressions.

This allows an interceptor to add custom keys to the body, or, modify existing ones if it wants to.

[dibyom]

#329

[wlynch]

Done!