Improve role fetching and unmarshaling

[espadolini]

This PR changes the default page size for ListRoles (that the API client uses by default, and by extent the cache initialization process) from 1000 to 100; roles can require a lot of effort during unmarshaling (as we validate expressions, which requires parsing them) and after a cluster event such as an auth server restart, a lot of nodes will all connect at the same time and get roles 1000 at a time, which can spike the memory used by the auth server. The default iteration page size of 1000 was picked somewhat arbitrarily and works well for resources that are just data with very little effort required during unmarshaling, but roles can be pretty big and hang around for a while, so tuning the page size down can help mitigate these memory requirement bursts.

In addition, this PR changes the version checking for roles to use jsoniter.Get to access the one field we need, rather than using encoding/json's unmarshaler for a whole resource header. A simple benchmark that unmarshals a reasonably "average" role with one where expression shows a 28% improvement in time spent in services.UnmarshalRoleV6, with a tiny reduction in allocations as well.

Benchmark result:

current
BenchmarkUnmarshalRole    210998             27651 ns/op           17377 B/op        331 allocs/op

new
BenchmarkUnmarshalRole    293476             19791 ns/op           16521 B/op        320 allocs/op

CPU profile flame graph for a relative comparison (with FastUnmarshal and ValidateRole being the same in both):

changelog: reduced memory and cpu usage after control plane restarts in clusters with a high number of roles

[espadolini]

For the record: the existing implementation sits at 27651 ns/op, this one with jsoniter.Get at 19791 ns/op, simply replacing json.Unmarshal with utils.FastUnmarshal (i.e. using jsoniter for a full ResourceHeader) is at 22823 ns/op - it's well worth it to apply the same approach to resources that are using json.Unmarshal but maybe not so much if they're already using utils.FastUnmarshal.

[public-teleport-github-review-bot]

@espadolini See the table below for backport results.

Branch	Result
branch/v14	Failed
branch/v15	Failed