feat: adds the capabilities for routers and offering speeds for direct link service

[ajay-malhotra1]

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This PR addresses the following changes to direct link service:
1. Add the capabilities for offering speeds which provides the metered and unmetered billing options for that offering speed
2. Add the capabilities for router representing the macsec and non-macsec capable router

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccIBMDLOfferingSpeedsDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -run TestAccIBMDLOfferingSpeedsDataSource_basic -timeout 700m
?   	github.com/IBM-Cloud/terraform-provider-ibm	[no test files]
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm	13.708s
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/hashcode	0.343s [no tests to run]
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/mutexkv	0.377s [no tests to run]
?   	github.com/IBM-Cloud/terraform-provider-ibm/version	[no test files]

$ make testacc TESTARGS='-run=TestAccIBMDLRoutersDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -run TestAccIBMDLRoutersDataSource_basic -timeout 700m
?   	github.com/IBM-Cloud/terraform-provider-ibm	[no test files]
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm	14.104s
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/hashcode	0.222s [no tests to run]
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/mutexkv	0.378s [no tests to run]
?   	github.com/IBM-Cloud/terraform-provider-ibm/version	[no test files]