Feat: Trust Domain URI parsing, complete rest of trust domain methods

[tjons]

This PR makes a number of changes:

• Implement code review feedback from @Coutlaw in feat: trust domain type string parsing and validation #5 by:
  • adding a well-known constant representing the spiffe protocol and the spiffe:// URI scheme,
  • refactoring the spiffeid.TrustDomain validation code to use switch() {}
• Add new features to support generating spiffeid.TrustDomain structs from std.Uri.
• Complete the rest of the necessary methods to provide effective parity with the Go SDK for spiffeid.TrustDomain.

After merging this PR, the spiffeid.TrustDomain type can be considered reasonably complete.

[Coutlaw]

looks good. I had a head scratching moment with your compare() function, but we can revisit that later. 👍