feat: trust domain type string parsing and validation

[tjons]

Implements the TrustDomain struct type from the go-spiffe API, and the following methods:

• TrustDomainFromString
• TrustDomain.name()
• TrustDomain.idString()
• TrustDomain.string()

This PR intentionally does not implement the URI-based types, which will be handled in the next iteration.

[Coutlaw]

great start for trust domains, already pulled this into my bundle code. Left 2 questions that don't need to be addressed now for this PR.

[Coutlaw]

❓ what would you say to something like this?

fn validateTrustDomain(idOrName: []const u8) InvalidTrustDomain!void {
    if (idOrName.len == 0) return InvalidTrustDomain.EmptyTrustDomain;

    for (idOrName, 0..) |character, index| {
        // TODO(tjons): use this to provide an index number for the invalid character
        _ = index;

        var other_allowed_character: bool = false;

        switch (character) {
            '@' => {
                return InvalidTrustDomain.TrustDomainContainsUserPart;
            },
            ':' => {
                return InvalidTrustDomain.TrustDomainContainsPortPart;
            },
            '%' => {
                return InvalidTrustDomain.TrustDomainContainsPercentEncodedCharacters;
            },
            '_', '-', '.' => {
                other_allowed_character = true;
            },
        }

        const digit = std.ascii.isDigit(character);
        const lowercase = std.ascii.isLower(character);

        if (!digit and !lowercase and !other_allowed_character) {
            return InvalidTrustDomain.TrustDomainContainsInvalidCharacters;
        }
    }

    return;
}

[Coutlaw]

100% up to you, I can't decide which is easier to read.

[tjons]

lol. this is much better, I didn't see the switch syntax yet...

[Coutlaw]

❓ should we have a global const for the uri base string length? since 9 is showing up a lot?
Not for this PR, but going forward maybe

[tjons]

this is a good point, will fix in the next iteration. I had defined this here: https://github.com/tjons/ziffe/blob/main/src/spiffeid/id.zig#L3 but forgot to use it.