feat: implement postgres kvbackend

[lyang24]

This commit implement a postgres backend to persist metasrv's data. A new enum BackendImpl with default to etcd was added to MetasrvOptions to activate postgres backend we could set BackendImpl to postgres and set store_addrs to postgres url.

I hereby agree to the terms of the GreptimeDB CLA.

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[killme2008]

@lyang24 Hi, there are some CI failures, could you check them?

[lyang24]

@lyang24 Hi, there are some CI failures, could you check them?

Yes, ci is green now

[killme2008]

@lyang24 Is it ready to review? If it is, I'd like to do it.

[killme2008]

Almost LGTM.

[WenyXu]

Rest LGTM

[killme2008]

@lyang24 Thanks a lot. It's a great work!

[killme2008]

@lyang24 Would like to add the sample configurations into https://github.com/GreptimeTeam/greptimedb/blob/main/config/metasrv.example.toml

and describe them in the doc https://docs.greptime.com/user-guide/operations/configuration#metasrv-only-configuration ?

[lyang24]

@lyang24 Would like to add the sample configurations into https://github.com/GreptimeTeam/greptimedb/blob/main/config/metasrv.example.toml

and describe them in the doc https://docs.greptime.com/user-guide/operations/configuration#metasrv-only-configuration ?

yes will do