Rename airflowctl assets get-by-alias to get-alias

[justinpakzad]

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The existing command to fetch an asset alias by it's ID was called get-by-alias, implying we are fetching an asset by it's alias. This is a bit misleading as it's actually fetching an asset alias by it's ID. This PR updates the name of the command to get-alias to better reflect what the command is actually doing. I have also updated the existing unit test and help text to align with the new naming/description.

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