feat(backup): S3 target supports optional endpoint_url (e.g. for Backblaze)

[tsgoff]

The S3 backup target can be used with any S3-compatible endpoint by optionally providing an endpoint_url parameter. This enables usage with services like Backblaze B2 Cloud Storage.

General information
This change extends the S3 backup target to support any S3-compatible storage by allowing an optional endpoint URL to be specified. This enables direct backup to providers like Backblaze B2 Cloud Storage, which offer an S3-compatible API.

Proposed changes
The S3 backup target now accepts an optional parameter.
If provided, this endpoint is used for all S3 operations, making it possible to use non-AWS S3-compatible storage (e.g., Backblaze B2).
The default behavior for AWS S3 remains unchanged if no endpoint is specified.
Expected behavior:
Users can configure the backup target to use any S3-compatible storage by specifying the appropriate endpoint URL.

Observed behavior before the change:
Only AWS S3 could be used, as the endpoint was not configurable.

How this patch changes the current behavior:
The patch adds flexibility by allowing the endpoint to be set, enabling support for additional S3-compatible providers.

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[tsgoff]

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