Derive Clone on Qdrant client

[timvisee]

Fixes #229

Makes the client Clone, as it is common in other clients too.

It wraps the internal channel pool in an Arc.

Ideally we'd Arc the configuration too. But, it is exposed, and doing so will change the API interface people might already rely on. Just cloning the channel pool is probably a good enough compromise.

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[coszio]

🙌