Add Delete Workflow API

[dbwiddis]

Description

Adds an API to delete a workflow.

It might be a good idea to warn users about losing track of provisioned resources and allow a "force" param. However, there are many more issues associated with resources (overwriting, etc.) and I think such extra logic belongs in an overall new strategy to handle this, so for now it's just a plain vanilla delete.

Issues Resolved

Fixes #23
Part of overall meta issue #88

Interactive testing

Without feature flag enabled (note this reqiures reading the workflow_id param, see #292):

localhost:9200/_plugins/_flow_framework/workflow/foobar

{
    "error": "This API is disabled. To enable it, update the setting [plugins.flow_framework.enabled] to true."
}

Enabled feature flag but no templates created:

{
    "error": "There are no templates in the global context."
}

Create a template, id qU00aowBH9rhI2IwnrH3

Delete it:

{
    "_index": ".plugins-ai-global-context",
    "_id": "qU00aowBH9rhI2IwnrH3",
    "_version": 2,
    "result": "deleted",
    "_shards": {
        "total": 1,
        "successful": 1,
        "failed": 0
    },
    "_seq_no": 2,
    "_primary_term": 1
}

Try to delete it again (it's not there):

{
    "_index": ".plugins-ai-global-context",
    "_id": "qU00aowBH9rhI2IwnrH3",
    "_version": 1,
    "result": "not_found",
    "_shards": {
        "total": 1,
        "successful": 1,
        "failed": 0
    },
    "_seq_no": 3,
    "_primary_term": 1
}

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