Documentation for the `aws_route_table` data source mentions that it supports a route table `id` as an argument, however it was missing from the actual provider code.
Adds in the missing provider code, adds a test, and updates the documentation to use `rtb_id` as the argument, instead of the more ambiguous `id`.
A VPC's main route table has an implicit subnet association, not an explicit subnet association. This caused a Terraform panic when using the `data_source_aws_route_table` resource to query the main route table for a VPC.
This fixes the Terraform panic, and allows the data lookup to complete successfully. Also added an acceptance test to verify the bugfix.
Fixes: #11134
Jake Champlin