…n fall back to original paths if ips not added yet (#64086)

Closes https://linear.app/sourcegraph/issue/SRC-453/modify-the-perforce-authorization-provider-to-support-ip-aware-sub

This PR builds on https://github.com/sourcegraph/sourcegraph/pull/64010, and enhances the re-insertion logic.

Before, we'd fail the sync operation outright if we tried to re-insert existing permissions that hadn't been converted from the path only form to the (ip, path) tuple.

Now, when trying to reinsert the existing permissions:

- if it has already been converted (`(path, ip)`) -> we save it back in the database using the UpsertWithUP() method
- if it hasn't been converted (`path` only) -> we save it back in the databse using the existing old plain Upsert method

I accomplish this by using a small interface that encpasulates the data and the insertion logic to use:

```go
type subRepoPermissionsUpserter interface {
	// UpsertWithSpec inserts or updates the sub-repository permissions with the data
	// stored in the upserter.
	UpsertWithSpec(ctx context.Context, store database.SubRepoPermsStore, userID int32, spec api.ExternalRepoSpec) error
}

type ipBasedPermissions struct {
	perms *authz.SubRepoPermissionsWithIPs
}

func (u *ipBasedPermissions) UpsertWithSpec(ctx context.Context, store database.SubRepoPermsStore, userID int32, spec api.ExternalRepoSpec) error {
	return store.UpsertWithSpecWithIPs(ctx, userID, spec, *u.perms)
}

type pathBasedPermissions struct {
	perms *authz.SubRepoPermissions
}

func (u *pathBasedPermissions) UpsertWithSpec(ctx context.Context, store database.SubRepoPermsStore, userID int32, spec api.ExternalRepoSpec) error {
	return store.UpsertWithSpec(ctx, userID, spec, *u.perms)
}

var (
	_ subRepoPermissionsUpserter = &ipBasedPermissions{}
	_ subRepoPermissionsUpserter = &pathBasedPermissions{}
)
```

Now, any code that deals with inserting sub repo permissions now can deal with a `[]subRepoPermissionsUpserter` without having to care about the exact semantics to use. Our re-insertion logic is also now more robust. 

## Test plan

New unit tests

## Changelog

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