Don't require trailing slash on issuer_uri

[dadgar]

🛠️ Description

Fixes an issue where we required a trailing slash on the issuer_uri. This is an unnecessary restriction as the issuer can include or not include it. For example the HCP issuer includes it but the TFC issuer doesn't.

https://hashicorp.atlassian.net/browse/HCPIE-809

🏗️ Acceptance tests

• Are there any feature flags that are required to use this functionality?
• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ TF_ACC=1 go test -v
=== RUN   TestAccServicePrincipalDataSource
--- PASS: TestAccServicePrincipalDataSource (10.84s)
=== RUN   TestAccServicePrincipalKeyResource
--- PASS: TestAccServicePrincipalKeyResource (42.48s)
=== RUN   TestAccServicePrincipalResource_Project
--- PASS: TestAccServicePrincipalResource_Project (21.88s)
=== RUN   TestAccServicePrincipalResource_ExplicitProject
--- PASS: TestAccServicePrincipalResource_ExplicitProject (11.78s)
=== RUN   TestAccServicePrincipalResource_Organization
--- PASS: TestAccServicePrincipalResource_Organization (13.29s)
=== RUN   TestAccWorkloadIdentityProviderResource
--- PASS: TestAccWorkloadIdentityProviderResource (19.05s)
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/provider/iam       119.773s