Support database secrets static roles

[krish7919]

This PR adds support for database secrets static roles as specified in the Vault documentation at https://www.vaultproject.io/api-docs/secret/databases.

Resolves #661

[codecov]

Codecov Report

Merging #662 (ae1dc4d) into develop (4b4532d) will increase coverage by 0.29%.
The diff coverage is 30.76%.

@@             Coverage Diff             @@
##           develop     #662      +/-   ##
===========================================
+ Coverage    83.49%   83.79%   +0.29%     
===========================================
  Files           59       61       +2     
  Lines         3103     3216     +113     
===========================================
+ Hits          2591     2695     +104     
- Misses         512      521       +9     

Impacted Files	Coverage Δ
hvac/api/secrets_engines/database.py	35.71% <30.76%> (-1.50%)	⬇️
hvac/api/auth_methods/jwt.py	92.30% <0.00%> (ø)
hvac/api/secrets_engines/pki.py	100.00% <0.00%> (ø)
hvac/api/auth_methods/approle.py	100.00% <0.00%> (ø)
hvac/constants/approle.py	100.00% <0.00%> (ø)
hvac/v1/__init__.py	86.58% <0.00%> (+0.27%)	⬆️
hvac/api/auth_methods/__init__.py	80.76% <0.00%> (+0.76%)	⬆️

[jeffwecan]

Thanks @krish7919!

I'll both merge this in, and see cutting a new release, this weekend. edit: Partially to give me some time to see about adding tests for these new methods. So if you care to give that a go in the meantime, feel free :D

[krish7919]

Merge please? :)