feat(spiffeid): accept case-insensitive scheme and normalize trust domain

[maxlambrecht]

Summary

Aligns SPIFFE ID parsing with SPIFFE spec:

• §2.4 SPIFFE ID parsing — Scheme and trust domain name are case-insensitive; path is case-sensitive. The parser now accepts mixed-case spiffe / authority input and canonicalizes the trust domain host to lowercase.
• §2.1 Trust domain — The authority host must be lowercase in the canonical form; we enforce that by normalizing after parse.
• §2.2 Path — Path rules unchanged; path bytes are preserved as provided.