support install external agent by Sermant-injector

[lilai23]

What type of PR is this?

Feature

What this PR does / why we need it?

support install external agent by Sermant-injector

Which issue(s) this PR fixes？

Fixes #1706

Does this PR introduce a user-facing change?

Yes. We will update user guide at sermant.io

Checklist

• Make sure there is a GitHub_issue related with this PR before you start working on it.
• Make sure you have squashed your change to one single commit.
• GitHub Actions works fine in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...njection/controller/SermantInjectorController.java	0.00%	26 Missing ⚠️

Flag	Coverage Δ	Complexity Δ
unittests	43.79% <0.00%> (-0.07%)	181.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ	Complexity Δ
...njection/controller/SermantInjectorController.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

... and 43 files with indirect coverage changes