Add CA fingerprint to agents configuration in 8.x versions

[jsoriano]

Try again something like bb84977 for 8.x stacks.

Adding this setting was evaluated and discarded in #789 because it didn't help configuring SSL in agents started by elastic-package stack up, as it was not supported in all versions of the stack supported by elastic-package. More about this can be read in #789.

The option implemented then and being used now is to install the certificate in the Elastic Agent containers, in /etc/ssl. This works well for Agents managed by elastic-package, but is not enough for external Elastic Agents, where installing the CA in the system may be more cumbersome.

Try to add the fingerprint back, as it would be helpful in some testing scenarios using external Elastic Agents.

Fix #1083.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-12-22T10:38:27.402+0000

• Duration: 33 min 32 sec

Test stats 🧪

Test	Results
Failed	0
Passed	867
Skipped	0
Total	867

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (34/34)	💚
Files	67.188% (86/128)	👍
Classes	61.957% (114/184)	👍
Methods	48.446% (374/772)	👎 -0.126
Lines	31.838% (3400/10679)	👎 -0.029
Conditionals	100.0% (0/0)	💚

[jsoriano]

Ah no, this setting cannot be set here, it would need to be set in the policies.
Not sure what we can do then from the elastic-package side for policies and agents configured manually.

[jsoriano]

Ah no, this setting cannot be set here, it would need to be set in the policies.

As @agithomas mentioned, this setting goes not in the policies, but in the output configuration. PR updated.

[agithomas]

Yes, it works. Tested with 8.5.1 stack and agent version

[agithomas]

Works well.

Recommend to add the documentation in the elastic-package about how to add the agent

example:
sudo ./elastic-agent install --certificate-authorities=$ELASTIC_PACKAGE_CA_CERT --fleet-server-es-ca=$ELASTIC_PACKAGE_CA_CERT --url=https://fleet-server:8220 --enrollment-token=