ntp: T9159: add source-address option for client requests

chrony (the daemon backing VyOS' NTP service) has no way to pin the
local address used for outgoing client requests to upstream servers.
"interface"/"listen-address" only configure the server/listening role
(binddevice/bindaddress); nothing controls the source address chosen
for packets chronyd itself originates.

On a router with more than one usable egress path to a given NTP
server (e.g. multiple transit/peering sessions), the kernel's default
source-address selection depends on whichever route wins at query
time. If that route happens to egress through an interface whose own
address is not globally reachable (a private peering-fabric segment,
for example), the request goes out with an address the reply can
never route back to - an intermittent, path-dependent failure with no
existing workaround at the NTP layer. BGP and friends already solve
the equivalent problem via "update-source" bound to the loopback;
NTP had no analogous option.

Add "service ntp source-address <address>", reusing the existing
source-address-ipv4-ipv6-multi include (same pattern already used for
e.g. RADIUS servers), and render it as chrony's "bindacqaddress"
directive - the client-side counterpart to "bindaddress" that chrony
already supports natively.

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