Migrate to a more easily extensible provider metadata struct

[joshraker]

Using a custom provider metadata struct allows us more flexibility to modify the struct such as using multiple clients.

[joshraker]

Acceptance tests passing

❯ make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 12 -timeout 30m
...
PASS
ok      github.com/aptible/terraform-provider-aptible/aptible   824.883s

[joshraker]

Still passing

❯ make testacc TEST_PARALLEL=24
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 24 -timeout 30m
...
PASS
ok      github.com/aptible/terraform-provider-aptible/aptible   512.996s