Add option for private PCUI deployment

Description

Add a VpcEndpointId parameter that, when specified, makes PCUI accessible only from within a VPC.

Move PCUI API Gateway from HTTP to REST in order to specify private endpoint configuration
Update URL to comply with stage name required by REST API (https://domain/resource is now https://domain/pcui/resource)
Use pcluster.svg for favicon as there was an issue with getting favicon when it isn't in root
Add a VpcEndpointId parameter that restricts access to within a VPC when set. The user must also supply subnets and security groups to place the Lambda function in an appropriate VPC.

Created stacks with and without VpcEndpointId parameter
Made requests from within VPC and from outside VPC to ensure correct access
Ran npm test
Ran pytest
Updated Lambda function and then created cluster with UI within private VPC on Nice DCV.

[ ] I added tests to new or existing code
[ ] I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
[ ] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
[ ] I made sure that any GitHub issue solved by this PR is correctly linked
[ ] I checked that infrastructure/update_infrastructure.sh runs without any error
[ ] I checked that npm run build builds without any error
[ ] I checked that clusters are listed correctly
[ ] I checked that a new cluster can be created (config is produced and dry run passes)
[ ] I checked that login and logout work as expected

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