Make providers a required field

[lidel]

This PR adds Pin.providers and PinStatus.providers as lists of max 20 multiaddrs.

• Pin.providers is now optional and can be empty
• PinStatus.providers is now mandatory and needs to have at least one multiaddr

The goal is to encourage pinning services to provide own multiaddrs in pin response under PinStatus.providers to ensure clients behind restrictive NAT can dial back and reach pinning service to facilitate data transfer.

As per https://github.com/ipfs/pinning-services-api-spec/issues/22#issuecomment-657788004 https://github.com/ipfs/pinning-services-api-spec/issues/22#issuecomment-657793428

Closes #22 cc @obo20

[lidel]

• Pin.providers is now optional and can be empty
• PinStatus.providers is now mandatory and needs to have at least one multiaddr

[lidel]

I believe this is good enough to be merged.