support pem in both base64 and plain text

[localvar]

certs in HTTP server spec were in plain text before but were updated to base64 encoding in #391 , this PR auto-detect the encoding, and the certs could be in either plain text or base64.

the PR also fix two issues in the AutoCertManager:

• When AutoCertManager is not started, autocertmanager.GetCertificate should return a nil error.
• If a domain is not managed by the AutoCertManager, autocertmanager.GetCertificate should return a nil error for that domain.

[codecov-commenter]

Codecov Report

Merging #425 (ec85066) into main (554efe7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #425   +/-   ##
=======================================
  Coverage   80.43%   80.43%           
=======================================
  Files          70       70           
  Lines        8093     8093           
=======================================
  Hits         6510     6510           
  Misses       1223     1223           
  Partials      360      360           

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