feat: support build-secrets in managed-mode

[tigarmo]

This is done by encoding the secrets, in the host-application, into the environment of the managed instance. The managed instance then decodes this environment and uses it to re-render the project secrets.

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[codecov]

Codecov Report

Merging #104 (ed5b524) into main (4c96df5) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head ed5b524 differs from pull request most recent head 0e69345. Consider uploading reports for the commit 0e69345 to get more accurate results

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   99.07%   99.11%   +0.04%     
==========================================
  Files          24       24              
  Lines         970     1015      +45     
  Branches      156      162       +6     
==========================================
+ Hits          961     1006      +45     
  Misses          6        6              
  Partials        3        3              

Files	Coverage Δ
craft_application/application.py	96.92% <100.00%> (+0.14%)	⬆️
craft_application/errors.py	100.00% <100.00%> (ø)
craft_application/secrets.py	100.00% <100.00%> (ø)

[cmatsuoka]

Implementation matches what was explained, overall this looks like a nice solution!