Using aws.StringValue and destructuring map with key value pair

[khanhtc1202]

What this PR does / why we need it:

Some minor fix on implementation of lambda cloud provider.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[nakabonne]

Thank you.
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.

[pipecd-bot]

Code coverage for golang is 32.94%. This pull request decreases coverage by -0.53%.

File	Function	Base	Head	Diff
pkg/app/piped/cloudprovider/lambda/client.go	percentageToPercent	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/deploy.go	deployExecutor.Execute	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/deploy.go	deployExecutor.ensureSync	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/deploy.go	deployExecutor.ensurePromote	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/deploy.go	deployExecutor.ensureRollout	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	Register	--	80.00%	+80.00%
pkg/app/piped/executor/lambda/lambda.go	findCloudProvider	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	loadFunctionManifest	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	sync	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	rollout	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	promote	--	0.00%	+0.00%
pkg/app/piped/executor/lambda/lambda.go	configureTrafficRouting	--	100.00%	+100.00%
pkg/app/piped/executor/lambda/lambda.go	build	--	0.00%	+0.00%
pkg/config/deployment.go	PipelineStage.UnmarshalJSON	45.90%	43.75%	-2.15%
pkg/app/piped/cloudprovider/lambda/client.go	client.GetTrafficConfig	0.00%	0.00%	+0.00%
pkg/app/piped/cloudprovider/lambda/client.go	client.UpdateTrafficConfig	0.00%	0.00%	+0.00%