This PR generally improves the error behavior of all monitors, and some specific ICMP related errors as well. These two items are combined in one PR because the general theme here is improving the ICMP error experience, and improving ICMP required improving all monitors.

Fixes #29346
and incremental progress toward #29692

General monitor improvements
Generally speaking, per #29692 we are trying to send monitor output to ES wherever possible. With this PR we now send any monitor initialization errors (such as a lack of ICMP kernel capabilities) during monitor creation to ES. We do this by allowing the monitor to initialize and run on schedule, even though we know it will always send the same error message. This lets users more easily debug issues in Kibana.

ICMP Specific Improvement
This PR also Removes broken a IP capability check that caused heartbeat to be unable to start. We now just rely on return codes from attempts to actually send packets. This is the more specific fix for #29346 . I was not able to exactly reproduce the exact customer reported issue, where the user somehow disabled ipv6 in a way that the ICMP loop that I can't exactly reproduce. I tried disabling ipv6 fully with sudo sysctl net.ipv6.conf.all.disable_ipv6=1 but that didn't yield the error in #29346

The logic is now simplified, there's no truly reliable way to know if you can send an ipv6 (or ipv4) ping before you send it (settings can change at any time! network cards can disappear!), so we just let the error codes happen as the check is executed. This is also generally a better UX in that the errors will now be visible in the Uptime app, not just the logs.

It should be noted that the ipv4 and ipv6 boolean options only are documented to affect how DNS lookups happen. With this change the behavior matches the docs.

Note that ICMP is a bit weird in that there's a single ICMP loop in heartbeat, and all monitors are really just interacting with that.

Removal of .synthetics
This also ignores the .synthetics folder which has been inconvenient for some time for devs, in that it dirties the git path

(cherry picked from commit 616db13)

Fixes broken macos python e2e test