[specification] Store enum fields as strings

[xyaoinum]

The current specification defines certain IG enum fields, such as seller capability, as byte sequences. This appears to be an implementation-level optimization, but it's unnecessary at the specification level.

Instead, the specification should directly use string types for these fields.

[qingxinwu]

it cannot be a simply string, due to the interest group size estimation part (step 9 of https://wicg.github.io/turtledove/#interest-group-estimated-size). Its size needs to be 4 bytes. So we defined a user defined "spec enum" type, which is different from IDL enum. See this discussion (https://github.com/WICG/turtledove/pull/1047/files#r1508216439) on the github pull request