#23207)

Flux HTTP and template fetching requests do not perform IP address
checks for local addresses. This behavior on the one hand allows SSRF
(Server Side Request Forgery) attacks via authenticated requests but on
the other hand is useful for scenarios that have legitimate requirements
to fetch from private addresses (eg, hosting templates internally or
performing flux queries to local resources during development).

To not break existing installations, the default behavior will remain
the same but a new --hardening-enabled option is added to influxd to
turn on IP address verification and limit both flux and template
fetching HTTP requests to non-private addresses. We plan to enable new
security features that aren't suitable for the default install with this
option.  Put another way, this new option is intended to be used to make
it easy to turn on all security options when running in production
environments. The 'Manage security and authorization' section of the
docs will also be updated for this option.

Specifically for flux, when --hardening-enabled is specified, we now
pass in PrivateIPValidator{} to the flux dependency configuration. The
flux url validator will then tap into the http.Client 'Control'
mechanism to validate the IP address since it is called after DNS lookup
but before the connection starts.

For pkger (template fetching), when --hardening-enabled is specified,
the template parser's HTTP client will be configured to also use
PrivateIPValidator{}. Note that /api/v2/stacks POST ('init', aka create)
and PATCH ('update') only store the new url to be applied later with
/api/v2/templates/apply. While it is possible to have InitStack() and
UpdateStack() mimic net.DialContext() to setup a go routine to perform a
DNS lookup and then loop through the returned addresses to verify none
are for a private IP before storing the url, this would add considerable
complexity to the stacks implementation. Since the stack's urls are
fetched when it is applied and the IP address is verified as part of
apply (see above), for now we'll keep this simple and not validate the
IPs of the stack's urls during init or update.

Lastly, update pkger/http_server_template_test.go's Templates() test for
disabled jsonnet to also check the contents of the 422 error (since the
flux validator also returns a 422 with different message). Also, fix the
URL in one of these tests to use a valid path.