Add SDK method for `addToken` and `removeToken`

[Levi-Ojukwu]

Description:

The contract has admin functions for token management that need SDK wrappers.

Why it matters: Token management is essential for multi-token support.

Acceptance Criteria:

• Create addToken(token, decimals) admin method
• Create removeToken(token) admin method
• Add tests

Relevant Files: contracts/invoice_liquidity/src/lib.rs:333-420, sdk/src/index.ts

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[thegreatfeez]

@thegreatfeez has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello, i would like to work on this issue

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This issue has been completed by @thegreatfeez as part of the Stellar Wave Program's 7th Wave 🥳

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