Add new `agent.version` condition

[jillguyonnet]

A new agent.version condition is introduced in package manifests in order to specify the minimum compatible version of Elastic Agent version.

The package registry should return this condition in order for Fleet to know the minimum compatible version of Elastic Agent version before installing the package.

package-registry/packages/package.go

Lines 119 to 122 in 6df12e8

	type Conditions struct {
	Kibana *KibanaConditions `config:"kibana,omitempty" json:"kibana,omitempty" yaml:"kibana,omitempty"`
	Elastic *ElasticConditions `config:"elastic,omitempty" json:"elastic,omitempty" yaml"elastic,omitempty"`
	}