ccl/sqlproxyccl: rename TenantResolver to tenant.Directory

[jaylim-crl]

Informs #76000.

ccl/sqlproxyccl: replace fallback resolution rule with a simple directory

Previously, --routing-rule is used as a fallback resolution if a tenant cannot
be found from the directory. Note that we no longer need this on Cloud. This
commit addresses a TODO comment by moving the resolution rule implementation
into a simple directory.

The plan is to replace the tenant.Resolver interface with a tenant.Directory
interface. The existing tenant.Directory struct will be renamed to
tenant.serviceDirectory. For now, there will be two types of directory
implementations: simple directory (a single rule), and service directory
(one that connects to a GRPC server). There are plans to add a static directory
(perhaps backed by a config file) which will enable roachprod to use sqlproxy
in a multi-tenant setup.

This TODO is also a pre-req to the load balancing work for sqlproxy. Without
that addressed, maintaining a routing rule for the balancing work can be
non-trivial.

Release note: None

ccl/sqlproxyccl: rename tenant.Directory to tenant.serviceDirectory

This commit addresses one of the TODOs to make tenant.Directory a generic
interface instead of a struct. For that to work, we'd have to rename the
existing Directory struct. This is purely a mechanical commit.

Release note: None

ccl/sqlproxyccl: add pkg/ccl/sqlproxyccl/tenant/{servicedir,simpledir}

Previously, all directory related files are located in a tenant package, which
can be confusing. This commit moves files into specific directories, and is
purely mechanical.

Release note: None

ccl/sqlproxyccl: rename tenant.Resolver to tenant.Directory

This commit completes the work of making tenant.Directory an interface.
Since we have renamed the previous Directory struct into a serviceDirectory
struct, we can now use Directory as an interface for what was previously known
as the resolver.

Release note: None

[cockroach-teamcity]

This change is 

[andy-kimball]

There's an existing confusion - we have two different definitions for Directory - is it the cache of directory entries, or is it the GRPC interface that the cache uses to populate itself? It makes sense to try and resolve that, however I'm not sure the way this PR does it is best. I'm worried about changing exported package names at this stage of the release. Won't we have a tricky transition to deal with across versions of CRDB? Why not leave tenant.DirectoryClient and tenant.DirectoryServer as-is, and then introduce a new tenant.DirectoryCache interface, or maybe tenantdir.Cache if we want it in another package?

[jaylim-crl]

Discussed offline. We'll make this DirectoryCache instead.

Closing this PR in favor of #78276.