Qualifier keys not validated

[matt-phylum]

packageurl-dotnet neither percent encodes qualifier keys nor prevents characters that require percent encoding from being present in qualifier keys. The PURL spec says that keys must not be encoded, but this is only possible if the implementation prohibits characters that would require encoding. For example, packageurl-dotnet will deserialize the PURL pkg:t/n?%23=%23 as having a qualifier #=# and then serialize it as pkg:t/n?#=%23, which will parse as having a subpath of =# instead of a qualifier.