[SPACES] default space should support /s/ api

[EthanStrider]

Describe the feature:
The default space is the only space that does not use the /s/<name> api approach

Describe a specific use case for the feature:
I understand why the default space uses the api format that it does, but it would make more sense if the default space supported both api approaches. Being agnostic would mean that the defaultspace is not a special case when writing automation for spaces in general.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

For reference, I think the relevant code is here:

kibana/x-pack/plugins/spaces/server/lib/request_interceptors/on_request_interceptor.ts

Line 33 in 91330d2

if (spaceId !== DEFAULT_SPACE_ID) {

We will have to update this to distinguish between requests made to the default space implicitly (i.e., without the /s/default identifier), and those made explicitly (i.e., with the /s/default identifier). We will want to rerwrite the url within the default space only if they're explicitly specifying it via /s/default, and skip the rewrite otherwise.

I haven't done any analysis downstream of this interceptor, but this should at least give us a starting point