Add disk_size_mb option to template build

[ya-luotao]

Summary

• Expose disk_size_mb keyword argument on Template.build and Template.build_in_background
• Forward as diskSizeMB to POST /v3/templates, only including the field when the caller provides it

Test plan

• Run Template.build(..., disk_size_mb: 4096) against a real environment and verify the request body contains diskSizeMB
• Confirm omitting the option keeps the request body unchanged (no diskSizeMB field)

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