SRC-15; Offchain Metadata Standard

[bitzoic]

Type of change

• New Standard

Changes

The following changes have been made:

• Adds the SRC-15 specification
• Adds the SRC-15 standard implementation
• Adds SRC-15 examples for single and multi asset

Related Issues

Closes #156

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.
• I have updated the changelog to reflect the changes on this PR.