Feature request: allow setting separate keepalive pool

[liuxiran]

Feature request

refer to apache/apisix#4506

it would be better to support keepalive pool settings via Dashboard

Describe the solution you'd like

• add corresponding configure fields to upstream form
• update https://github.com/apache/apisix-dashboard/blob/master/api/conf/schema.json to match apisix 2.8

• apisix-dashboard/api/internal/core/entity/entity.go

  Line 164 in fb78963

  type UpstreamDef struct {

[liuxiran]

closed by #2117