Implement jwtsvid.SVID type and validation

[azdagron]

This issue is to implement the jwtsvid.SVID type as outlined in svid.go along with code to parse and validate a JWT-SVID according to the specification.

The SVID type should contain the raw JWT-SVID token as well as denormalized convenience fields for various claims, like the SPIFFE ID (i.e. sub), audience (i.e. aud) and a bucket for the other claims.

There should be two methods:

1. ParseInsecure - simply parses a token into the SVID type but does not validate. This is mainly useful when the token is received from a trust source (like the workload API)
2. ParseAndValidate - parses and validates the JWT-SVID token according to the spec and returns the SVID type.

This issue depends on #57 for validation.

[azdagron]

Fixed in #84