lightning/br/dumpling: add session-token to external storage url (#12947)

[ti-chi-bot]

This is an automated cherry-pick of #12947

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What is changed, added or deleted? (Required)

add session-token to external storage url

Now we use the iam user with its access key id and secret access key to access s3. But the best practice from aws is using the iam role with its access ket id, secret access key and session token to access s3. Because the session token is time-limited.

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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): lightning: access aws resource by session token tidb#40753

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[qiancai]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4ab47ac