Check for custom endpoint

[DyfanJones]

Issue regarding custom endpoint not correctly being parse when provided from config file:

For example:

library(paws)
client <- s3(config(credentials(profile = "minio"), signature_version = "s3v4"))
client$generate_presigned_url("list_objects", params = list(Bucket = "demo"))
#> [1] "http://demo.127.0.0.1:9000/?X-Amz-Algorithm=..."

^{Created on 2024-04-02 with reprex v2.1.0}

However it should be the following:

library(paws)
client <- s3(config(credentials(profile = "minio"), signature_version = "s3v4"))
client$generate_presigned_url("list_objects", params = list(Bucket = "demo"))
#> [1] "http://127.0.0.1:9000/demo?X-Amz-Algorithm=..."

^{Created on 2024-04-02 with reprex v2.1.0}

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.26%. Comparing base (1c363d1) to head (47751de). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #768 +/- ## ======================================= Coverage 85.25% 85.26% ======================================= Files 213 213 Lines 15136 15145 +9 ======================================= + Hits 12904 12913 +9 Misses 2232 2232 ```

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