Implement spiffeid package

[azdagron]

The spiffeid package provides types for the SPIFFE ID and Trust Domain. Some of the functionality has been implemented as part of the V2 scaffolding. However, there are two remaining methods: Parse and ParseURI. These two methods are in charge of parsing a SPIFFE ID from a string or URI. They should perform proper validation of the URI, ensuring that it is well formed according to the SPIFFE ID specification.

In addition, the trust domain portion of the SPIFFE ID should be normalized to lower-case to facilitate SPIFFE ID comparison.

Unit-tests for all functionality in the spiffeid package should be introduced as part of implementing this issue.