Expose TBS TTL config via integration policy

[carsonip]

Tail based sampling TTL sampling.tail.ttl config is only configurable via apm-server.yml, and is not available to fleet-managed apm-server. TTL is a useful knob to adjust TBS storage usage if storage limit is reached and sampling.tail.storage_limit cannot be further increased for any reason.

Add TTL config to integration policy, APM UI, and docs.

Files requiring changes (and possibly more):

• https://github.com/elastic/integrations/blob/c2ceb26e216b88ec373e599f9c38948594a6b570/packages/apm/agent/input/template.yml.hbs#L71
• https://github.com/elastic/kibana/blob/c5a00472cba967608fc6ba8b47eb7d3deee94780/x-pack/plugins/observability_solution/apm/public/components/fleet_integration/apm_policy_form/settings_definition/tail_sampling_settings.tsx#L99
• https://github.com/elastic/observability-docs/blob/e1279a3edabc9ec2d90e5f041bbcc599bddbec66/docs/en/observability/apm/configure/sampling.asciidoc#L65