taprpc+multi: concert `taprpc` into a versioned Go module

[bhandras]

With this PR, taprpc can now be used in other projects without also pulling in the entire taproot-assets repository as a dependency. This modularization is desirable because it reduces unnecessary dependencies, leading to cleaner and more efficient codebases. Additionally, it allows for more focused and manageable updates, improves build times, and enhances the reusability of taprpc in various contexts.