vault: Carve out fetching of Issuing CA into a function

[draychev]

The goal of this PR is to carve out the function figuring out the Issuing CA for the connected Hashicorp Vault in a separate function.

This obviously adds more code and still does the same thing. The idea is to eventually implement pki/revoke in the Hashi Vault API via the ReleaseCertificate function, so that the temp certificate does not linger around Vault and is never renewed etc.

The pki/revoke API call to Hashicorp Vault will happen as part of this GitHub Issue: #2068

ref #2070 #507

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Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?

[codecov-io]

Codecov Report

Merging #2069 (9ef22e9) into main (e0513d3) will increase coverage by 0.10%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##             main    #2069      +/-   ##
==========================================
+ Coverage   57.81%   57.91%   +0.10%     
==========================================
  Files         140      140              
  Lines        5781     5786       +5     
==========================================
+ Hits         3342     3351       +9     
+ Misses       2437     2433       -4     
  Partials        2        2              

Impacted Files	Coverage Δ
...certificate/providers/vault/certificate_manager.go	56.17% <70.00%> (+7.98%)	⬆️
pkg/catalog/proxy.go	100.00% <0.00%> (ø)
pkg/catalog/catalog.go	100.00% <0.00%> (ø)
pkg/catalog/debugger.go	97.22% <0.00%> (+0.07%)	⬆️

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