Allow the target allocator to use a custom serviceaccount

[jaronoff97]

With the addition of increased target-allocator capabilities, from this PR, i tried running the latest build but got some errors related to service accounts. Right now the target allocator CR only uses the default service account. It is unable to use a service account either injected via config or created by the operator. This makes it so that it crash loops when trying to do discovery

{"level":"error","ts":1650641805.262842,"logger":"allocator","msg":"Pod failure","component":"opentelemetry-targetallocator","error":"pods is forbidden: User \"system:serviceaccount:opentelemetry:default\" cannot list resource \"pods\" in API group \"\" in the namespace \"opentelemetry\"","stacktrace":"main.configureFileDiscovery\n\t/app/main.go:154\nmain.newServer\n\t/app/main.go:121\nmain.main\n\t/app/main.go:55\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:250"}

Could we change the target allocator CR to match what the collector does for its serviceaccounts?