chore(logs): add String() string methods on complex types logged

[qdm12]

Issue summary

When reviewing most of the log usage in #1928 I noticed several structures did not have an associated .String() string method, which makes the log ugly and bit pointless (e.g. who cares about a pointer address).

The following structures were identified as logged out, complex enough and without a String method:

• dot.LogConfig
• dot.NetworkConfig
• dot.Config
• dot.RPCConfig
• types.GrandpaForcedChange
• GrandpaMessage
• CommitMessage
• VoteMessage
• runtime.Context
• PublicKey (mirror Hex with String)
• babe.Slot done in chore(logging): replace log15 library #1928
• dot.LogConfig
• types.GrandpaScheduledChange
• scale.VaryingDataType
• types.EpochData
• types.GrandpaForcedChange
• types.ConfigData
• types.Block
• types.Header
• cfg.Core.Roles
• lib/crypto.PublicKey
• crypto.Keypair