Add iteration support to etcd

[rosstimothy]

Introduces a new Items(context.Context, backend.IterateParams) iter.Seq2[backend.Item, error] to the etcd backend. The core logic for this function was moved from GetRange, and GetRange now defers to calling Iterate and collecting a slice of items.

The motivation for this change is to attempt to reduce the complexity of pagination for consumers of GetRange. While backend.IterateRange and backend.StreamRange do already exists to serve the same purpose, they too have suffered from pagination bugs in the past. Additionally, this aims to unify iteration to using the iter package instead of the various homegrown iteration mechanisms that exist throughout the code base.

There is also one distinct difference to the iteration api: it allows retrieving items within the specified range in ascending or descending key order. While there may not be many use cases for this today, exposing this in the api from it's inception is easier than adding functional options, or IterateAscending/IterateDescending later.

[rosstimothy]

I attempted to determine the performance impact of this change by running a modified version of BenchmarkGetNodes to run against etcd and for the sake of time a single node count.

goos: darwin
goarch: arm64
pkg: github.com/gravitational/teleport/lib/services/local
cpu: Apple M2 Pro
                                       │ master.txt  │            new.txt            │
                                       │   sec/op    │   sec/op     vs base          │
GetNodes/tt(nodes=2000)-12   178.5m ± 2%   177.3m ± 1%  ~ (p=0.739 n=10)

                                       │  master.txt  │               new.txt               │
                                       │     B/op     │     B/op      vs base               │
GetNodes/tt(nodes=2000)-12   11.83Mi ± 4%   12.35Mi ± 2%  +4.37% (p=0.000 n=10)

                                       │ master.txt  │              new.txt               │
                                       │  allocs/op  │  allocs/op   vs base               │
GetNodes/tt(nodes=2000)-12   148.0k ± 0%   148.0k ± 0%  +0.03% (p=0.002 n=10)```

[espadolini]

If we want to follow https://pkg.go.dev/iter#hdr-Naming_Conventions we should call the method Range or something like that, shouldn't we?

[fspmarshall]

LGTM once error handling is improved.