feat(templates): cache templates based on etag

[ecrupper]

go-github supports adding a custom transport layer for caching, and given that we already have a cache service and that templates have a very obvious cache key (fetch URL), this felt like a no brainer to spare a lot of SCM calls.

[codecov]

Codecov Report

❌ Patch coverage is 66.99029% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.05%. Comparing base (541d690) to head (3a37b66).

Files with missing lines	Patch %	Lines
compiler/registry/github/content_transport.go	75.47%	7 Missing and 6 partials ⚠️
cache/redis/opts.go	0.00%	5 Missing ⚠️
cache/redis/store_template.go	50.00%	2 Missing and 2 partials ⚠️
cmd/vela-server/server.go	0.00%	4 Missing ⚠️
cache/redis/get_template.go	72.72%	2 Missing and 1 partial ⚠️
cache/redis/extend_template.go	60.00%	1 Missing and 1 partial ⚠️
api/admin/settings.go	0.00%	1 Missing ⚠️
api/pipeline/template.go	0.00%	1 Missing ⚠️
cache/cache.go	0.00%	1 Missing ⚠️

❌ Your project check has failed because the head coverage (58.05%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1465      +/-   ##
==========================================
+ Coverage   58.03%   58.05%   +0.02%     
==========================================
  Files         663      667       +4     
  Lines       25617    25703      +86     
==========================================
+ Hits        14867    14923      +56     
- Misses      10049    10069      +20     
- Partials      701      711      +10     

Files with missing lines	Coverage Δ
cache/flags.go	100.00% <ø> (ø)
cache/redis/redis.go	84.50% <ø> (ø)
cache/setup.go	100.00% <100.00%> (ø)
compiler/native/native.go	77.88% <100.00%> (ø)
compiler/registry/github/github.go	92.59% <100.00%> (-7.41%)	⬇️
database/pipeline/table.go	100.00% <ø> (ø)
scm/github/webhook.go	90.32% <ø> (ø)
api/admin/settings.go	0.00% <0.00%> (ø)
api/pipeline/template.go	0.00% <0.00%> (ø)
cache/cache.go	43.75% <0.00%> (-2.92%)	⬇️
... and 6 more

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