feat: api adjustments - extension methods to use sdk easier

[wregulski]

This pull request introduces new functionalities and improvements to the compat and script packages, including new methods for signing messages and extracting script parts, as well as corresponding tests.

New Functionalities:

• compat/bsm/sign.go: Added SignMessageString function to sign messages and return the signature as a base64-encoded string.
• script/script.go: Added GetParts and GetPublicKey methods to the Script type for extracting decoded chunks and public keys from scripts.

Code Improvements:

• compat/bsm/sign.go: Imported encoding/base64 package to support the new SignMessageString function.
• script/script.go: Imported github.com/pkg/errors package for improved error handling.

Tests:

• script/script_test.go: Added tests for GetParts and GetPublicKey methods, covering various scenarios including valid, invalid, and edge cases.

Linked Issues / Tickets

No references to specific tasks (maintenance/feedback).

• I have added new unit tests
• All tests pass locally
• I have tested manually in my local environment

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have updated CHANGELOG.md with my changes
• I have run npm run doc and npm run lint one final time before requesting a review