feat: add new resource for keys api

[jakedipity]

Introduces a new logdna_key resource to interact with the keys api to manage service and ingestion keys.

This change includes some additional upkeep changes:

• Update github.com/hashicorp/terraform-plugin-sdk/v2 to v2.15.0
• Remove duplicate code around existence testing
• Have requests use the http client attached to the provider instead of creating a new one each time
• Ensure all tests respect the env API_URL
• Fix memory reference error when testing existence of non-existent resource

Ref: LOG-12483 Signed-off-by: Jacob Hull jacob@planethull.com