CLI requires HTTPS OIDC issuer, authorize, and token URLS

[cfryanr]

This PR is to address #1012.

Release note:

CLI returns an error for non-HTTPS `--issuer` URL, and for non-HTTPS auto-discovered OIDC authorization and token endpoints from that issuer.

[codecov]

Codecov Report

Merging #1013 (7946731) into main (cc50fc9) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1013      +/-   ##
==========================================
- Coverage   79.37%   79.36%   -0.02%     
==========================================
  Files         133      133              
  Lines        9931     9944      +13     
==========================================
+ Hits         7883     7892       +9     
- Misses       1784     1786       +2     
- Partials      264      266       +2     

Impacted Files	Coverage Δ
pkg/oidcclient/login.go	89.76% <100.00%> (-0.59%)	⬇️

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