[apm integration] validate supported TLS protocol versions

Supported TLS protocol versions is currently an unverified text-entry field, which allows users to enter incorrect values and prevent their apm-server from starting:


These values should be validated before allowing a user to save an invalid config.

Questions:
- We've defined the default (valid) values for [supported protocol versions](https://github.com/elastic/apm-server/blob/main/apmpackage/apm/manifest.yml#L140-L143). Is there support at the package level to provide an allowlist of all valid values, something like `allowed: [...]`?
- If not, would this be then best implemented in the apm-ui?