Centralise vault environment variable name definitions

[chris-j-h]

Create constants for the Azure and Hashicorp environment variable names and add to EnvironmentVariables.java.

All environment variables in use by Tessera will now be contained within the same file. To change the value of one of the names, they need only be changed in this central location.

[codecov-io]

Codecov Report

Merging #651 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #651      +/-   ##
============================================
- Coverage     99.47%   99.47%   -0.01%     
  Complexity     1937     1937              
============================================
  Files           302      302              
  Lines          5947     5943       -4     
  Branches        283      283              
============================================
- Hits           5916     5912       -4     
  Misses            5        5              
  Partials         26       26

Impacted Files	Coverage Δ	Complexity Δ
...ey/vault/azure/AzureKeyVaultClientCredentials.java	100% <100%> (ø)	8 <0> (ø)	⬇️
...hashicorp/HashicorpKeyVaultServiceFactoryUtil.java	96.66% <100%> (-0.4%)	10 <6> (ø)
...a/key/vault/azure/AzureKeyVaultServiceFactory.java	92.85% <100%> (ø)	5 <1> (ø)	⬇️
...ult/hashicorp/HashicorpKeyVaultServiceFactory.java	100% <100%> (ø)	10 <1> (ø)	⬇️

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