Add auth_methods module to support AppRole

[discogestalt]

This PR deprecates the methods for AppRole authentication in the base client and adds an AppRole class to auth_methods. It includes tests and full docstrings for the class.

[krish7919]

I was looking for this... can we please review this one? :)

[krish7919]

@jeffwecan Can we assign this to someone for review please?

[jeffwecan]

For sure! I'll aim to get this reviewed over the next day or so.

[jeffwecan]

Some (relatively) unrelated integration test cases appear to be failing here. I'm still working on tracking down exactly what is occurring there...

[jeffwecan]

Sorry for the extended delay getting back to this PR! @discogestalt I pushed up a tweak to your approle additions here: bd353a8. Most all our auth login() method default to setting the client's token attribute to the one sent back in an authentication response. In this case, the divergence made itself known through that unrelated JWT auth test case failure.

I'm still tracking down some other issues but will aim to get them squared away today.

[codecov]

Codecov Report

Merging #637 into develop will increase coverage by 0.51%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #637      +/-   ##
===========================================
+ Coverage    83.49%   84.01%   +0.51%     
===========================================
  Files           59       61       +2     
  Lines         3103     3203     +100     
===========================================
+ Hits          2591     2691     +100     
  Misses         512      512              

Impacted Files	Coverage Δ
hvac/api/auth_methods/__init__.py	80.76% <100.00%> (+0.76%)	⬆️
hvac/api/auth_methods/approle.py	100.00% <100.00%> (ø)
hvac/constants/approle.py	100.00% <100.00%> (ø)
hvac/v1/__init__.py	86.58% <100.00%> (+0.27%)	⬆️