feat: Specify subnet id list and allow selection of availability zone

[roehrijn]

This PR:

• refactors the determination of the machine's subnet ID by moving all related code to the GetSubnet() function instead of having it partly in Create() and GetSubnetID().
• allows the user to specify more than a single subnet id. If more than one is specified, the least occupied subnet is choosen.
• allows the determination of subnets based on devpod:devpod tag without providing a VPC ID. I think this was a bug. The code required to have a VPC ID specified in this case, however it didn't do anything with the value in this decision branch.
• allows the user to specify an availability zone. Subnets are than filtered by this zone. This feature is for example meant to provide a workaround to get a workspace up and running even during a zone outage on AWS side.

_{Jan Roehrich jan.roehrich@mercedes-benz.com, Mercedes-Benz Tech Innovation GmbH, legal info/Impressum}

[bkneis]

Thanks for the contribution @roehrijn! This looks very useful. I left just a couple of minor remarks / questions. Could you also comment on the PR a brief description of how you tested these changes. Many thanks

[roehrijn]

Could you also comment on the PR a brief description of how you tested these changes. Many thanks

of course, going to add that soon. What I can say so far: I prepared resources in AWS accounts for every situation which could happen while traversing GetSubnet(). In other words, I think I covered every branch in that function during my tests.

[roehrijn]

How I tested:

1. specify a single subnet id through AWS_SUBNET_ID -> Subnet is choosen :heavy_check_mark:
2. specify three subnet ids through AWS_SUBNET_ID -> least occupied subnet is choosen :heavy_check_mark:
3. specify six subnet ids, 2 per AZ, through AWS_SUBNET_ID + set availibility zone through AWS_AVAILABILITY_ZONE -> least occupied subnet in desired availability zone is choosen :heavy_check_mark:
4. having two VPCs, each with 3 subnets in one account. Subnets of one VPC are tagged with devpod:devpod. No AWS_... environment variables except credentials -> least occupied tagged subnet is choosen :heavy_check_mark:
5. having two VPCs, each with 3 subnets in one account. Subnets of one VPC are tagged with devpod:devpod. Availability zone set through AWS_AVAILABILITY_ZONE -> subnet in desired AZ is choosen :heavy_check_mark:
6. having two VPCs, each with 3 subnets in one account. Subnets of one VPC are tagged with devpod:devpod. VPC ID set through AWS_VPC_ID -> tagged subnet in desired VPC is choosen :heavy_check_mark:
7. having two VPCs, one with tagged subnets, one default VPC with public IP assignment enabled subnets. VPC ID of default VPC is given through AWS_VPC_ID -> least occupied subnet in default VPC with public IP assignment enabled is choosen :heavy_check_mark: