[Fleet] Adding integration: List of strings does not allow value ['*']

[simitt]

Kibana version:
master

Describe the bug:
When defining a package spec with type: string and multi: true, defining a (default) value of ['*'] is forbidden.
This needs to be allowed, as e.g. it is required for APM allowed origins.

Steps to reproduce:
0. Check out elastic/apm-server#4690

1. Run APM package defined in [apmpackage] Add config options supported in ESS apm-server#4690 (e.g. by running apm-integration-testing with command ./scripts/compose.py start master --no-apm-server --with-package-registry --package-registry-apm-path=<your-path-to-apm-server>/apmpackage/apm --with-elastic-agent)
2. Try to Install APM integration with ['*'] as value for RUM - Allow Origins.

Expected behavior:
Any kind of string should be allowed.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[skh]

It seems we're running into the rarely-used YAML feature of aliases here, which start with a *, see

http://blogs.perl.org/users/tinita/2019/05/reusing-data-with-yaml-anchors-aliases-and-merge-keys.html

When the YAML parser encounters the single * it complains about the missing alias name which should come directly after it.

If we ever want to use & we'll run into a similar problem

I can prepare a fix where we escape * and &, but I'm not sure this is the correct behaviour -- what if we do want to use YAML aliases after all?

[nchaulet]

I think we should probably escape the * we already do it for number string we could probably have a better escaping function here: https://github.com/elastic/kibana/blob/master/x-pack/plugins/fleet/server/services/epm/agent/agent.ts/#L93-L97