core - add ability to add custom functions to jmespath

[thisisshi]

Adds the ability to include custom jmespath functions for use anywhere we express jmespath queries

This came up in the finops slack where there was a question around copy-related-tag(s) from a lambda function to its corresponding cloudwatch log group. The Cloudwatch log group follows the convention of /aws/lambda/$function_name but there's no way to do that in jmespath currently. There are enhancement proposals/issues here:

• New function: array[string] split(string $separator, string $subject) jmespath/jmespath.site#35
• [Proposal] add "split" function - array[string] split(string $separator, string $subject) jmespath/jmespath.jep#26
• Function proposal - split() jmespath/jmespath.jep#3
• substring support jmespath/jmespath.py#167
• Spec extension - string slice jmespath/jmespath.jep#5

jmespath's python library includes a way to include custom functions via its jmespath.functions.Functions class, which is utilized in this PR.

extant usage of jmespath should be preserved, as we're utilizing functools.partial to inject the additional options into the call, so extant call sites should continue to behave as normal and new jmespath.search and jmespath.compile.search calls should also be fine.

policies:
  - name: copy-related-tag-with-split
    resource: aws.log-group
    filters:
      - type: value
        key: logGroupName
        value: "/aws/lambda/"
        op: in
        value_type: swap
    actions:
      - type: copy-related-tag
        resource: aws.lambda
        key: "split(`/`, logGroupName)[-1]"
        tags: "*"

[ajkerrigan]

Love it. I'm all for giving jmespath some more smarts and this seems like a reasonable way to handle it 👍 . I know the idea of extending jmespath from the c7n side has come up before, with split() and slicing being recurring patterns. Thanks for jumping on this.

[kapilt]

seems cool, but needs docs, else its not discoverable.

[thisisshi]

@kapilt updated with docs and test

[kapilt]

technically mailer doesn't have a runtime dependency on c7n, so we should just leave the default here afaics.

[kapilt]

lgtm, one minor