Add support for rebalancing endpoint

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Is there an existing issue for this?

• [X] I have searched the existing issues

Is your feature request related to a problem? Please describe.

There is no library support for the rebalancing functionality: https://alpaca.markets/docs/api-references/broker-api/rebalancing/

Describe the solution you'd like.

No response

Describe an alternate solution.

No response

Anything else? (Additional Context)

No response