Resurrect _xpack/watcher routes

[joegallo]

Related to #51816 / #68905.

Adds back the _xpack/watcher routes when using rest compatibility for a request.

# no change here
joegallo@galactic:~/Code/elastic/elasticsearch $ curl http://localhost:9200/_watcher/stats
{"_nodes":{"total":1,"successful":1,"failed":0},"cluster_name":"runTask","manually_stopped":false,"stats":[{"node_id":"oPIII6_ZS4eP9-wUj3l8NA","watcher_state":"started","watch_count":0,"execution_thread_pool":{"queue_size":0,"max_size":0}}]}%

# _xpack/watcher doesn't exist
joegallo@galactic:~/Code/elastic/elasticsearch $ curl http://localhost:9200/_xpack/watcher/stats
{"error":"no handler found for uri [/_xpack/watcher/stats] and method [GET]"}%

# unless...
joegallo@galactic:~/Code/elastic/elasticsearch $ curl -H'accept:application/json;compatible-with=7' http://localhost:9200/_xpack/watcher/stats
{"_nodes":{"total":1,"successful":1,"failed":0},"cluster_name":"runTask","manually_stopped":false,"stats":[{"node_id":"oPIII6_ZS4eP9-wUj3l8NA","watcher_state":"started","watch_count":0,"execution_thread_pool":{"queue_size":0,"max_size":0}}]}%

[joegallo]

I copied over all the watcher API definitions into xpack equivalents, and then I copied over the basic yml tests and got them passing. On the one hand, this is a lot of code change, but on the other hand it's not like I sat down and wrote it all by hand either.

I could certainly do a round of tree shaking if we think it's important to cut this down -- that is, we could have a larger yml file of many tests that attempts to get something approaching reasonable coverage, rather than this copy/paste/edit cycle that I've currently done.

[jakelandis]

LGTM. No worries about the cp/paste from the original tests (assuming they are stable :) ) This does drive home the point we need to add the v7 into the sourceset (directory) name (which is on my todo list).