support dynamic install external agent by Sermant Backend

[lilai23]

What type of PR is this?

Feature

What this PR does / why we need it?
Support dynamic install external agent by Backend
(1) publish dynamic install command

(2) view the external agent status and version

(3) view the events for external agent reported

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 56.00000% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...io/sermant/backend/service/HotPluggingService.java	12.50%	5 Missing and 2 partials ⚠️
...ant/backend/entity/heartbeat/HeartbeatMessage.java	0.00%	4 Missing ⚠️

Flag	Coverage Δ	Complexity Δ
unittests	43.88% <56.00%> (+<0.01%)	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
...nt/backend/entity/heartbeat/ExternalAgentInfo.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...ermant/backend/entity/hotplugging/CommandType.java	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
.../backend/entity/hotplugging/HotPluggingConfig.java	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
...ant/backend/entity/heartbeat/HeartbeatMessage.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...io/sermant/backend/service/HotPluggingService.java	65.00% <12.50%> (-27.86%)	0.00 <0.00> (ø)

... and 13 files with indirect coverage changes