infra: centralize shared dependencies in [workspace.dependencies]

[Icahbod]

The root Cargo.toml defines a workspace containing only apexchainx_calculator but leaves no [workspace.dependencies] block. Each member crate re-declares its own soroban-sdk version (currently 21.0.0), which works for a single-crate workspace but risks silent version drift as soon as sibling crates land (e.g. apexchainx_payment_escrow, apexchainx_settlement). Centralise common dependencies in a [workspace.dependencies] section at the root and have the calculator inherit via soroban-sdk = { workspace = true }. Include a [workspace.package] block for edition, license, and repository metadata so future members pick these up automatically.

[MicD746]

Hello Maintainer, please can I work on this issue and give you a better result

[gloskull]

Hello, I have 4 years of experience delivering solutions for projects like this, covering backend, frontend, and smart contract development. After carefully reviewing your requirements, I have a clear understanding of the task and am confident in my ability to deliver a clean, efficient, and thoroughly tested solution on time. I prioritize code quality, maintainability, and industry best practices throughout the development process.
My PR will be submitted in under 10hours.

[gloskull]

Done with this issue.
Please merge and give and high review.
Please if there are any more tasks, assign them to me, i'd make sure to get them done quickly and submit my PR's in under 5 hours