Apply agent partition to load services and agent api

[huikang]

Description

OSS split of ent PR

Testing & Reproduction steps

• load service in OSS -> ent (manually tested)

1. Register a service in oss client agent, e.g., C-1
2. Stop C-1
3. Replace C-1's binary with an ent binary
4. Add non-partition to C-1's config file and join the cluster
5. The service should be loaded from C-1's on-disk service config

• Agent endpoint

1. Start a client agent in non-default partition
2. register a service at the client agent without partition field in service definition

Links

#15881
#15963

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PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern