[Fleet] Add ability to force install while creating agent policy

[hop-dev]

Since the addition of package verification in 8.4 (#133822), an unverified package can only be installed if the force flag is supplied.

When creating an agent policy, we install the agent integration if it isnt alread, and if sys_monitoring is enabled then we also install the system integration. Currently, there is no way to force install these integrations during agent policy creation so the agent policy will always fail.

The workaround is to force install the packages before creating the agent policy. This is interfering with our API integration tests as all regustry packages are unverified, e.g I have had to skip this test.

[elasticmachine]

Pinging @elastic/fleet (Feature:Fleet)

[nchaulet]

@jen-huang Yes those tests are still skipped, I think using package fixture will be the same issue as you cannot install an upload a package when creating a policy.

I think the fix mark proposed here is still relevant to have a flag to force install or ignore verification errors when creating a policy.