secrets: add support for URLOpeners to all drivers

[vangent]

This is mostly mechanical, copying the pattern @zombiezen established for blob.

The interesting parts are probably:

• I made localsecrets register for 2 schemes, 'stringkey://" and "base64key://".
• vault is a bit interesting since the URL contains the Vault server address + token; I made it Dial but cache the result.

Updates #1174.

[codecov]

Codecov Report

Merging #1427 into master will increase coverage by 0.28%.
The diff coverage is 82.88%.

@@            Coverage Diff             @@
##           master    #1427      +/-   ##
==========================================
+ Coverage    76.9%   77.19%   +0.28%     
==========================================
  Files          58       58              
  Lines        4517     4621     +104     
==========================================
+ Hits         3474     3567      +93     
- Misses        872      875       +3     
- Partials      171      179       +8

Impacted Files	Coverage Δ
secrets/localsecrets/localsecrets.go	95.23% <100%> (+3.93%)	⬆️
secrets/gcpkms/kms.go	74.13% <60%> (-7.45%)	⬇️
secrets/awskms/kms.go	80% <75%> (-5.37%)	⬇️
secrets/vault/vault.go	92.59% <90.9%> (-2.15%)	⬇️
blob/fileblob/fileblob.go	80.28% <0%> (+0.56%)	⬆️
pubsub/mempubsub/mem.go	85.55% <0%> (+2.22%)	⬆️
secrets/secrets.go	92.3% <0%> (+7.69%)	⬆️

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[vangent]

Gentle ping @clausti ?