feat: support external service

[lingsamuel]

Signed-off-by: Ling Samuel lingsamuelgrace@gmail.com

Type of change:

• New feature provided

What this PR does / why we need it:

First part of #927
Proposal is here: #927 (comment)

This feature is considered unstable because how to implement some features (schemes and ports) is still under discussion.

Doc PR is incoming.

[tao12345666333]

This is a new implementation of #726 and #927 , PTAL.

/cc @elvis-cai @purekeeper @BrandonArp @NMichas

[codecov-commenter]

Codecov Report

Merging #1306 (d90912b) into master (5f2c398) will increase coverage by 0.41%.
The diff coverage is 34.89%.

❗ Current head d90912b differs from pull request most recent head 762e488. Consider uploading reports for the commit 762e488 to get more accurate results

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
+ Coverage   40.47%   40.88%   +0.41%     
==========================================
  Files          78       78              
  Lines        7076     7232     +156     
==========================================
+ Hits         2864     2957      +93     
- Misses       3897     3934      +37     
- Partials      315      341      +26     

Impacted Files	Coverage Δ
pkg/providers/apisix/translation/apisix_plugin.go	57.54% <ø> (ø)
pkg/providers/apisix/translation/translator.go	0.00% <ø> (ø)
pkg/providers/utils/string.go	0.00% <0.00%> (ø)
...kg/providers/apisix/translation/apisix_upstream.go	48.83% <35.29%> (-51.17%)	⬇️
pkg/providers/apisix/translation/apisix_route.go	25.02% <35.83%> (+5.88%)	⬆️

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[lingsamuel]

All tests passed

[tao12345666333]

could you please resolve conflicts? thanks