Add a minimal SRP client API

[nand-nor]

Description

Adds logic to expose minimal functionality for supporting SRP service registration (supports acting as client only). Also adds an example bin as well as a slight reworking of the examples dir to match the same style as present in the esp-hal crate. If accepted, this will be the first in a series of PRs working on this general area of functionality.

I opted to build the openthread libs with SRP functionality and include them precompiled, even though the SRP client functionality (as exposed via the publicly accessible Openthread object's methods) is gated via feature flag. This has the downside of making the openthread libs bigger even though a user of the esp-openthread crate may not want to ever use SRP. Do you have any thoughts on this @bjoernQ @jessebraham or preferences as to how this is handled? If this was a bad choice I can restructure the build.rs file in esp-openthread-sys to handle pulling in different libraries at build time, and include multiple copies of the various libs based on compile-time feature flags.

Testing

Tested with a v1.3 Border Router and esp32c6, running the srp-client-example bin. When running, should see logs like the following showing successful service registration and add proxying as output in the otbr-agent logs:

otbr-agent[1289574]: [INFO]-ADPROXY-: Advertise SRP service updates: host=ot-esp32.default.service.arpa.
otbr-agent[1289574]: [INFO]-ADPROXY-: Handle publish SRP host 'ot-esp32.default.service.arpa.': OK
otbr-agent[1289574]: [INFO]-ADPROXY-: Handle publish SRP service 'ot-service._ipps._tcp.default.service.arpa.': OK
otbr-agent[1289574]: 7d.09:39:43.782 [I] SrpServer-----: Handler result of SRP update (id = 1960774133) is received: OK
otbr-agent[1289574]: 7d.09:39:43.782 [I] SrpServer-----: Committing update for new host ot-esp32.default.service.arpa.
otbr-agent[1289574]: 7d.09:39:43.782 [I] SrpServer-----:     Granted lease:7200, key-lease:680400, ttl:7200
otbr-agent[1289574]: 7d.09:39:43.782 [I] SrpServer-----: Add new service 'ot-service._ipps._tcp.default.service.arpa.'
otbr-agent[1289574]: 7d.09:39:43.782 [I] SrpServer-----: Send success response with granted lease: 7200 and key lease: 680400
otbr-agent[1289574]: 7d.09:39:45.209 [I] SrpServer-----: Received DNS update from fd2e:c69b:fa93:1:d42a:bd6b:6419:29b
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----: Processed SRP update info
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----:     Host:ot-esp32.default.service.arpa.
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----:     Lease:7200, key-lease:1209600, ttl:7200
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----:     1 host address(es):
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----:       fd2e:c69b:fa93:1:d42a:bd6b:6419:29b
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----:     Adding service 'ot-service._ipps._tcp.default.service.arpa.'
otbr-agent[1289574]: 7d.09:39:45.220 [I] SrpServer-----: SRP update handler is notified (updatedId = 1960774134)

Also checked that add proxying is working; Im able to discover the service on the LAN using the service discovery app on my android phone, should also work with avahi-browse and/or dns-sd

[bjoernQ]

Awesome! Thank you

This has the downside of making the openthread libs bigger even though a user of the esp-openthread crate may not want to ever use SRP.

I think it's fine - the precompiled libraries are quite small and luckily, we don't need to compile them to multiple architectures here. If that ever becomes a problem, we can revisit this but for now this is definitely not a problem