rpc server: listen to ipv6 socket if available and --experimental-rpc-endpoint CLI option

[niklasad1]

Close #3488, #4331

This changes/adds the following:

1. The default setting is that substrate starts a rpc server that listens to localhost both Ipv4 and Ipv6 on the same port. Ipv6 is allowed to fail because some platforms may not support it
2. A new RPC CLI option --experimental-rpc-endpoint which allow to configure arbitrary listen addresses including the port, if this is enabled no other interfaces are enabled.
3. If the local addr is not found for any of the sockets the server is not started throws an error.
4. Remove the deny_unsafe from the RPC implementations instead this is an extension to allow different polices for different interfaces/sockets such one may enable unsafe on local interface and safe on only the external interface.

So for instance in this PR it's now possible to start up three RPC endpoints as follows:

$ polkadot --experimental-rpc-endpoint "listen-addr=127.0.0.1:9944,rpc-methods=unsafe" --experimental-rpc-endpoint "listen-addr=0.0.0.0:9945,rpc-methods=safe,rate-limit=100" --experimental-rpc-endpoint "listen-addr=[::1]:9944,optional=true"

Needs to be addressed

1. Support binding to a random port if it's fails with the default stuff for backward compatible reasons
2. How to sync that the rpc CLI params and that the rpc-listen-addr align, hard to maintain...
3. Add similar warning prints for exposing unsafe methods on external interfaces..
4. Inline todos + the hacky String conversion from rpc params.

Cons with this PR

Manual strings parsing impl more error-prone than relying on clap....

//cc @jsdw @BulatSaif @PierreBesson @bkchr

[PierreBesson]

make it possible to use different polices for each listening interface with a special notation which is very similar to an URL which is ip:addr/?rpc-methods=safe&cors=*

This is definitely very useful for multiple use cases and it is a feature that has been lacking for:

• Validator/Collator security hardening: having 2 listen addresses with different permissions: one for regular queries (eg. sync state check) and one for admin tasks (eg. setting up keys)
• RPC node: having 2 listen addresses, one public with rate limits and CORS and one internal which allows unsafe RPC calls.

[jsdw]

$ polkadot --rpc-listen-addrs "127.0.0.1:9944:/?rpc-methods=unsafe, 0.0.0.0:9945:/?rpc-methods=safe&rpc-rate-limit=100"

This format for allowing options to be passed with multiple listen addrs looks good to me!

[BulatSaif]

Very useful feature. One remark: can we avoid using whitespace in the arguments? It sometimes causes issues. Ideally, allow specifying multiple --rpc-listen-addr arguments.

[niklasad1]

Very useful feature. One remark: can we avoid using whitespace in the arguments? It sometimes causes issues. Ideally, allow specifying multiple --rpc-listen-addr arguments.

Yeah, it should work without whitespaces but I can add a test for it.

[lexnv]

LGTM! Nice job here 🙏