clients/v1: add service access token methods

[jac]

Adds the RPC wrapper methods for the Service Access Token create/list/revoke RPCs.

Updates the introspect wrapper to also return UserID.

Part of CORE-980

Test plan

Ran locally

create

(*sams.CreateServiceAccessTokenResponse)(0x140004000f0)({
 Token: (*clientsv1.ServiceAccessToken)(0x1400040a360)(id:"ca16cf7e-f011-4690-b335-a3c9c028db2c" service:"analytics" scopes:"analytics::analytics::read" user_id:"0196aa9f-a46c-7589-91e0-5bb9e13960e9" display_name:"dev-token"),
 Secret: (string) (len=95) "sams_at_bdoFwJDN_-c-814WR2RERm4UghVnLMhdNCiQUYsC1Uo.xqDS_dq3BRnrWdacWEsSD7zFa0SQ5YEtfRQX8mT_zYE"
})

list

([]*clientsv1.ServiceAccessToken) (len=1 cap=1) {
 (*clientsv1.ServiceAccessToken)(0x140001b2360)(id:"ca16cf7e-f011-4690-b335-a3c9c028db2c" service:"analytics" scopes:"analytics::analytics::read" user_id:"0196aa9f-a46c-7589-91e0-5bb9e13960e9" display_name:"dev-token")
}

introspect

(*sams.IntrospectTokenResponse)(0x140003ce0c0)({
 Active: (bool) true,
 Scopes: (scopes.Scopes) (len=1 cap=1) {
  (scopes.Scope) (len=26) "analytics::analytics::read"
 },
 ClientID: (string) (len=45) "sams_cid_01970868-ae38-7ad1-82a0-73ddacaba597",
 ExpiresAt: (time.Time) 1970-01-01 00:00:00 +0000 UTC, // zero value as no expiry
 UserID: (string) (len=36) "0196aa9f-a46c-7589-91e0-5bb9e13960e9"
})

revoke
doesn't have a response value so here is the result of introspect again for the revoked token

(*sams.IntrospectTokenResponse)(0x140003ce0c0)({
 Active: (bool) false,
 Scopes: (scopes.Scopes) {
 },
 ClientID: (string) "",
 ExpiresAt: (time.Time) 1970-01-01 00:00:00 +0000 UTC,
 UserID: (string) ""
})

[unknwon]

Docstrings are now all outdated 😁 assuming this is an intended change.