feat: allow session pool settings in connection url

[olavloite]

Allows min/max sessions for the backing SessionPool to be specified in the connection URL.

Towards googleapis/java-spanner-jdbc#334

[codecov]

Codecov Report

Merging #821 (4bdabb6) into master (2f1450c) will decrease coverage by 0.07%.
The diff coverage is 59.25%.

@@             Coverage Diff              @@
##             master     #821      +/-   ##
============================================
- Coverage     85.05%   84.98%   -0.08%     
- Complexity     2564     2580      +16     
============================================
  Files           143      143              
  Lines         14062    14132      +70     
  Branches       1348     1368      +20     
============================================
+ Hits          11961    12010      +49     
- Misses         1539     1550      +11     
- Partials        562      572      +10     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/spanner/SessionPoolOptions.java	70.31% <30.95%> (-19.35%)	19.00 <3.00> (+3.00)	⬇️
...le/cloud/spanner/connection/ConnectionOptions.java	89.13% <88.57%> (+1.87%)	77.00 <8.00> (+12.00)
...m/google/cloud/spanner/connection/SpannerPool.java	86.88% <100.00%> (+2.44%)	31.00 <0.00> (ø)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.24% <0.00%> (+0.19%)	73.00% <0.00%> (ø%)
...n/java/com/google/cloud/spanner/SessionClient.java	92.06% <0.00%> (+0.79%)	16.00% <0.00%> (+1.00%)

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