[Fleet] Add ability to set a proxy for agent binary source

[hop-dev]

Summary

Closes #162862

Add a proxy selector for agent download sources.

Selecting a proxy will set the agent.download.proxy_url field on the agent policy (this can be checked in the "view full agent policy" flyout). I have marked this functionality as beta as we currently can only use the URL of the proxy, not the certificates or headers.

Editing a proxy should automatically update all associated policies which use download sources with that proxy.

Deleting a proxy should remove the proxy from all associated policies.

Verifying the field is set...

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)

[apmmachine]

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
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[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 44d13db

Failed CI Steps

• Serverless Security Cypress Tests #9
• Serverless Security Investigations Cypress Tests #1

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	1023.1KB	1.0MB	+1.6KB

History

• 💔 Build #151428 failed 5370445
• 💔 Build #151393 failed 788a3e5

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @hop-dev

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[afharo]

SO changes LGTM

[nchaulet]

🚀