Create @ipfs/specs-stewards team

@ipfs/specs-stewards team was created as part of https://github.com/ipfs/specs/pull/289

We need to decide who should belong to the team, sharing responsibility of triaging and reviewing improvement proposals (#289).

My initial idea is to add past/recent lead implementation maintainers from existing stewards teams, namely:
- https://github.com/orgs/ipfs/teams/w3dt-stewards/members
- https://github.com/orgs/libp2p/teams/w3dt-stewards/members
- https://github.com/orgs/ipld/teams/w3dt-stewards/members
- https://github.com/orgs/multiformats/teams/w3dt-stewards/members

Starting with a smaller group should be enough to get us started.
Over time, we will add trusted leads from other organizations maintaining IPFS implementations.

## TODO

- [ ] discuss: add / remove stewards
- [ ] figure out how to automate management of the group (JSON under https://github.com/ipfs/github-mgmt/ ?)