Parse target URIs as RFC 3986 (if flagged)

[jdcormie]

Brings grpc-java in compliance with the grpc name syntax standard. Please review each commit individually -- I do not plan to squash.

This is technically a breaking change because there's a set of target strings that previously parsed fine under java.net.URI but will now throw. There are 2 known types:

• IPv6 scopes in the authority must now be prefixed by %25 not just %. Unlikely to cause problems because no known NameResolver looks for an IP address in the authority (dns puts it in the path).
• Square brackets in the query/fragment are no longer tolerated. Unlikely to cause problems because no known NameResolver uses query/fragment.

Even though both of these are unlikely, we guard the behavior change behind the GRPC_ENABLE_RFC3986_URIS flag (off by default). Technically NameResolverProvider is a public API so there could be resolvers out in the world that we haven't considered.