Implement entitlement verification and AWS metering

[stefanprodan]

This PR adds entitlement verification and metering for AWS Marketplace service.

Ref: controlplaneio-fluxcd/distribution#88

PS. When the operator is not installed from the AWS Marketplace, an offline entitlement client is employed that generates
a perpetual license token.

[souleb]

It would be nice to have a test for this. I believe we can use WithEndpointResolverV2() for that purpose.

type endpointResolverV2 struct {
	URL url.URL
}

func (e *endpointResolverV2) ResolveEndpoint(ctx context.Context, params marketplacemetering.EndpointParameters) (smithyendpoints.Endpoint, error) {
	return smithyendpoints.Endpoint{
		URI: e.URL,
	}, nil
}

func TestMarketplaceClient() {
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		...
	}))
	defer srv.Close()
	opt := marketplacemetering.WithEndpointResolverV2(&endpointResolverV2{URL: url.URL{Scheme: "http", Host: srv.URL}})
	mc := marketplacemetering.NewFromConfig(cfg, opt)
	...
}

[stefanprodan]

Mocking the AWS APIs could come later after we release a version with metering and we tests it on EKS.

[stefanprodan]

I've tested this PR on EKS with an IRSA for AWSMarketplaceMeteringRegisterUsage and all works, the usage is registered and the resulting token is valid 🎉

[souleb]

LGTM!