fix: ensure baseUrl has trailing / for proper URL creation

[TylerHillery]

What kind of change does this PR introduce?

Bug fix, related to #8 the URL doesn't get created properly when leaving off a trailing / in the baseUrl

What is the current behavior?

If you leave off a trailing / in thebaseUrl the last path gets chopped off due to how new URL(path, baseUrl) works which gets called in buildUrl e.g. for list namespaces

baseUrl = "https://example.com/storage/iceberg"
url = "https://example.com/storage/v1/warehouse/namespaces" // iceberg gets removed

What is the new behavior?

New behavior always ensures the baseUrl ends with a trialing / so it will function the same if you pass in with or without a trailing /

baseUrl = "https://example.com/storage/iceberg"
url = "https://example.com/storage/iceberg/v1/warehouse/namespaces" // iceberg stays

Additional context