feat(sql-backend): disable etcd meta store initialization when sql backend is enabled

[yezizp2012]

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What's changed and what's your intention?

As title, to enable sql-backend user can configure two options as follows:

1. --sql-endpoint [endpoint address]
2. --backend sql

For example, the endpoint address could be postgres://postgres:@localhost:5432/postgres for PG and sqlite://posts.sqlite?mode=rwc for SQLite.

Besides, I will support adapt it in risedev in another PR.

Checklist

• I have written necessary rustdoc comments
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[kwannoel]

LGTM, thank you!