fix: restore go toolkit build

[Luzijano]

Summary

• fixes go.mod so the module uses a CI-compatible Go directive (1.20 instead of patch-form 1.21.6)
• removes duplicate SDKConfig declarations and the undefined object / misspelled Merkle proof types that prevented package compilation
• fixes the checked-in example so go test ./... also compiles the example package
• keeps the address tests passing by decoding mocked API responses and preserving the existing empty-UTXO behavior for non-array response shapes

Validation

• RED before fix: go test ./... failed immediately on the invalid Go directive, then exposed compile errors after the directive was corrected
• GREEN after fix: go test ./...
• git diff --check

Bounty / improvement program

Submitted as a small build/test reliability improvement under BitgesellOfficial/bitgesell#81.

Assisted by Hermes Agent.

[MyTH-zyxeon]

Review assist for maintainers:

This is a broader build-restoration patch: it normalizes the Go directive, removes duplicate SDKConfig definitions, fixes the checked-in example import/constructor usage, restores missing interface/type definitions, and starts decoding address API responses instead of returning empty structs.

Suggested merge checks:

• Run go test ./... from a clean checkout with the intended Go toolchain.
• Confirm the BaseAPIURL normalization in NewAddress preserves callers that pass either the root API URL or the full /bgl/v1/blockchain path.
• Review GetAddressUTXO: it currently returns an empty AddressUTXOs on decode errors while other address methods surface decode errors; decide whether that compatibility behavior is intentional.
• Check public type compatibility around TransactionMerkelProof -> TransactionMerkleProof, the new object alias, and the removed duplicate SDKConfig declarations before merging.

I do not see wallet/key, signing, broadcast, live RPC, or live-chain behavior changes in this patch as written; the acceptance criterion is that the toolkit compiles/tests from a clean checkout without unintentionally changing SDK error/API compatibility.