feat: Use TopicStats to implement getSplitBacklog

[palmere-google]

We use the TopicStatsService to compute the backlog size for the
assigned splits in the PubsubLiteUnbounded Reader.

We will refresh backlog information every ten seconds, and will use
stale information for up to a minute if the topic stats serivce is
unavailable.

Constructing the TopicBacklogReader in SubscriberOptions is a bit
awkward, but I wanted to make sure we don't need to re-resolve the
subscription -> topic mapping when we split or merge the source.
Otherwise we end up with a hard dependency on the admin service.

[codecov]

Codecov Report

Merging #228 into master will decrease coverage by 0.47%.
The diff coverage is 43.75%.

@@             Coverage Diff              @@
##             master     #228      +/-   ##
============================================
- Coverage     72.71%   72.23%   -0.48%     
+ Complexity      752      746       -6     
============================================
  Files           139      135       -4     
  Lines          3987     4009      +22     
  Branches        209      207       -2     
============================================
- Hits           2899     2896       -3     
- Misses          965      991      +26     
+ Partials        123      122       -1     

Impacted Files	Coverage Δ	Complexity Δ
...oud/pubsublite/beam/PubsubLiteUnboundedSource.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...oogle/cloud/pubsublite/beam/SubscriberOptions.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...oud/pubsublite/beam/PubsubLiteUnboundedReader.java	72.95% <91.30%> (+4.26%)	24.00 <4.00> (+4.00)
.../java/com/google/cloud/pubsublite/ProjectPath.java	0.00% <0.00%> (-60.00%)	0.00% <0.00%> (-4.00%)
...n/java/com/google/cloud/pubsublite/TopicPaths.java	77.41% <0.00%> (-22.59%)	7.00% <0.00%> (+5.00%)	⬇️
...in/java/com/google/cloud/pubsublite/TopicPath.java	66.66% <0.00%> (-20.84%)	2.00% <0.00%> (-2.00%)
...java/com/google/cloud/pubsublite/LocationPath.java	66.66% <0.00%> (-20.84%)	2.00% <0.00%> (-2.00%)
.../com/google/cloud/pubsublite/SubscriptionPath.java	66.66% <0.00%> (-20.84%)	2.00% <0.00%> (-2.00%)
...com/google/cloud/pubsublite/SubscriptionPaths.java	90.90% <0.00%> (-9.10%)	8.00% <0.00%> (+6.00%)	⬇️
...in/java/com/google/cloud/pubsublite/TopicName.java	66.66% <0.00%> (-8.34%)	2.00% <0.00%> (-1.00%)
... and 16 more

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[dpcollins-google]

You'll also have to integrate the changes made to the topic path creation logic in this pr.

[palmere-google]

Okay, rebased to include the path changes

PTAL