feat: use external template for 3PP plugin generator

[mdonnalley]

What does this PR do?

Utilize https://github.com/salesforcecli/plugin-template-sf-external for generating 3PP plugins

What issues does this PR fix or reference?

@W-11781515@

[cristiand391]

QA notes:

2pp plugin

🟢 can build it, run UTs and NUTs.
🟢 description is saved to pjson
🟢 env display hook works
🟡 name validator only allows plugin-[a-z], can't use plugin-deploy-retrieve, plugin-internal-test, etc.

3pp plugin
sf dev generate plugin (external)
sf dev generate command execute:apex`

🟢 can build it, run UTs and NUTs.
🟢 description is saved to pjson
🟢 code cov choice is respected (selected 90%, removed a command UT, nyc throws:

ERROR: Coverage for lines (84.61%) does not meet global threshold (90%)
ERROR: Coverage for functions (50%) does not meet global threshold (90%)
ERROR: Coverage for branches (0%) does not meet global threshold (90%)
ERROR: Coverage for statements (84.61%) does not meet global threshold (90%)