Use dedicated Revision type instead of primitive int

[ikhoon]

Currently, a revision is represented as an int.

centraldogma-python/centraldogma/dogma.py

Line 140 in 6098f1a

revision: Optional[int] = None,

It would be nice to use a dedicated type introduced in #14

centraldogma-python/centraldogma/data/revision.py

Lines 20 to 34 in 6098f1a

	class Revision:
	def __init__(self, major):
	self.major = major
	@staticmethod
	def init() -> Revision:
	return _INIT
	@staticmethod
	def head() -> Revision:
	return _HEAD
	_INIT = Revision(1)
	_HEAD = Revision(-1)