Implement header method

[Regis-Caelum]

Issues/PR

Fix Issue #6.

Changes

• Add Header struct according to protobuf defination on Extended Header response.
• Implement header method.
• Update headerpath method.
• Add Tests for Header method.

Files affected

• types.go
• const.go - (heightKey is no longer user)
• client.go
• client_test.go

[tzdybal]

@Regis-Caelum please rebase or merge with main.