Implement ConfigReload on full config SET update

[donggyu-nexthop]

Why I did it

Resolves sonic-net/sonic-buildimage#22362

The current "full config update" operation over gNMI silently fails.

This is because the operation relied on GNOI System Reboot to pick up the requested configuration and apply it; however, the proper reboot call implementation ripped out the configuration file handling to better align with the GNOI system specs. This left the config reload operation non-functional.

How I did it

client.MixedDbClient has a new field which can optionally store "post-response callbacks", which are just functions that will be invoked after the gRPC response has been issued back to the client.

During SetFullConfig, this field will be set to a callback that invokes DBusClient::ConfigReload(). Then, at the gNMI server layer, the server will invoke MixedDbClient::RunCallback() to execute the callback (if registered).

The reason this step had to be deferred until the very end of the request lifecycle was because the ConfigReload D-Bus endpoint will restart ALL services, including the gNMI server, which means that calling this function directly from within SetFullConfig leaves the gNMI client hanging with no response.

How to verify it

• prep a testbed with changes from this branch
• ssh into testbed, and use gnmic to issue SET commands over gnmi
  • example query:

gnmic -a "<DUT ip>:<DUT port>" --insecure set \
--encoding json_ietf \
--delete "sonic-db:/CONFIG_DB/localhost/" \
--update-path "sonic-db:/CONFIG_DB/localhost/" \
--update-file <path to json with new config db>

• then on the DUT, verify the changes were successful

# if you modified the description of ethernet 4 port
sonic-db-cli hget "PORT|Ethernet4" description

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Implement gNMI config reload without relying on gNOI reboots

Link to config_db schema for YANG module changes

N/A for this PR

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

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[ronan-nexthop]

@donggyu-nexthop can you use git blame and tag people who may be suitable to review here. I think because no reviewers are auto-assigned people may not be activly looking at this.

@sneelam20 perhaps a good example of why the @gnmi-dev alias would be good to have auto-added to this, or alternatively, some form of auto-assigning based on git blame

[donggyu-nexthop]

@ganglyu @abdosi @jipanyang @ndas7 could you give this a look? thank you!

[anders-nexthop]

@sneelam20 @hdwhdw can we get some reviewers assigned to look at this? I do not have permissions to add reviewers.

This PR was discussed in the 5/14 UMF meeting. sonic-gnmi used to support a whole-file config replace option, which was invoked by passing the filename to the rpc as an UPDATE. The original implementation relied on an early version of GnoiReboot which did a 'config reload' rather than a hard reboot. This feature broke when an actual GnoiReboot impl was added. This PR plumbs the feature correctly, using DBUS to call 'config reload' directly.

This is not the same usage as the save-on-set feature, which triggers 'config save' after every write to make config changes persistent. This approach is intended for whole-config operations instead, where the user wants to bulk-load a config throw away anything pre-existing, then reload the system so that configuration takes effect.