fix: specify user_agent when download sample.

[ryota-sakamoto]

Issue #, if available:

Fix #103

Description of changes:

If user_agent is not specified, the server response 403 error as follows.

$ RUST_LOG=debug cargo run -- bootstrap
...
[2023-05-12T17:38:28Z DEBUG dy::bootstrap] temporary download & unzip directory: TempDir { path: "/var/folders/ys/zr1z7ql10kvcp8v3g2zbyp080000gs/T/.tmpvtdrOq" }
Temporarily downloading sample data from https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/samples/sampledata.zip
[2023-05-12T17:38:28Z DEBUG reqwest::connect] starting new connection: https://docs.aws.amazon.com/
Error: ReqwestError(reqwest::Error { kind: Status(403), url: Url { scheme: "https", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("docs.aws.amazon.com")), port: None, path: "/amazondynamodb/latest/developerguide/samples/sampledata.zip", query: None, fragment: None } })

If user_agent is specified, the download is succeeded.

[2023-05-12T17:41:46Z DEBUG dy::bootstrap] temporary download & unzip directory: TempDir { path: "/var/folders/ys/zr1z7ql10kvcp8v3g2zbyp080000gs/T/.tmpypvWPL" }
Temporarily downloading sample data from https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/samples/sampledata.zip
[2023-05-12T17:41:46Z DEBUG reqwest::connect] starting new connection: https://docs.aws.amazon.com/
[2023-05-12T17:41:47Z DEBUG dy::bootstrap] Downloading the file at: /var/folders/ys/zr1z7ql10kvcp8v3g2zbyp080000gs/T/.tmpypvWPL/downloaded_sampledata.zip
[2023-05-12T17:41:47Z DEBUG dy::bootstrap] Finished writing content of the downloaded data into '/var/folders/ys/zr1z7ql10kvcp8v3g2zbyp080000gs/T/.tmpypvWPL/downloaded_sampledata.zip'

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