Add support for slack mrkdwn formatting

[Dev25]

Allow end users to have a BODY_TEMPLATE to contain slack's markup formatting as per their docs.

This is enabled by default however if there are concerns about it we can add a config flag to make it opt in?

Example use case is using code blocks in the notifications:

        env:
        - name: BODY_TEMPLATE
          value: |
            Event: {{ .EventString }}
            {{ if and (ne .EventType "commit") (gt (len .Commits) 0) }}
            {{ range .Commits }}
            * {{ call $.FormatLink (print $.VCSLink "/commits/" .Revision) (truncate .Revision 7) }}: `{{ .Message }}`
            {{end}}{{end}}
            {{ if (gt (len .EventServiceIDs) 0) }}Resources updated:
            ```{{ range .EventServiceIDs }}
            * {{ . }}
            {{ end }}```{{ end }}
            {{ if gt (len .Errors) 0 }}Errors:
            ```{{ range .Errors }}
            Resource {{ .ID }}, file: {{ .Path }}:

            > {{ .Error }}
            {{ end }}```{{ end }}

[oliviabarrick]

Welcome @Dev25! It looks like this is your first PR to justinbarrick/fluxcloud 🎉

[oliviabarrick]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[oliviabarrick]

Hi @Dev25. Thanks for your PR.

I'm waiting for a justinbarrick member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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