pageserver: API updates for sharding (#6330)

The theme of the changes in this PR is that they're enablers for #6251
which are superficial struct/api changes.

This is a spinoff from #6251:
- Various APIs + clients thereof take TenantShardId rather than TenantId
- The creation API gets a ShardParameters member, which may be used to
configure shard count and stripe size. This enables the attachment
service to present a "virtual pageserver" creation endpoint that creates
multiple shards.
- The attachment service will use tenant size information to drive shard
splitting. Make a version of `TenantHistorySize` that is usable for
decoding these API responses.
- ComputeSpec includes a shard stripe size.

libs/compute_api/src/spec.rs

@@ -75,6 +75,10 @@ pub struct ComputeSpec {
     pub remote_extensions: Option<RemoteExtSpec>,
 
     pub pgbouncer_settings: Option<HashMap<String, String>>,
+
+    // Stripe size for pageserver sharding, in pages
+    #[serde(default)]
+    pub shard_stripe_size: Option<usize>,
 }
 
 /// Feature flag to signal `compute_ctl` to enable certain experimental functionality.