feat: add autoConfigEmulator connection option

[olavloite]

Adds autoConfigEmulator connection option. When this option is set to true for a connection:

1. The connection will by default try to connect to localhost:9010 (unless a specific host/port is set in the connection URL).
2. Plain text communication will be enabled.
3. Authentication will be disabled.
4. The instance and database in the connection string will automatically be created on the emulator if any of them do not yet exist. Any existing instance and/or database will remain untouched.

Towards googleapis/java-spanner-jdbc#380

[codecov]

Codecov Report

Merging #931 (6d6676f) into master (213dddc) will increase coverage by 0.01%.
The diff coverage is 94.23%.

@@             Coverage Diff              @@
##             master     #931      +/-   ##
============================================
+ Coverage     85.21%   85.22%   +0.01%     
- Complexity     2650     2658       +8     
============================================
  Files           145      146       +1     
  Lines         14358    14404      +46     
  Branches       1391     1395       +4     
============================================
+ Hits          12235    12276      +41     
- Misses         1540     1545       +5     
  Partials        583      583              

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/spanner/connection/ConnectionImpl.java	84.04% <0.00%> (-0.33%)	180.00 <0.00> (ø)
.../google/cloud/spanner/connection/EmulatorUtil.java	97.22% <97.22%> (ø)	3.00 <3.00> (?)
...le/cloud/spanner/connection/ConnectionOptions.java	90.53% <100.00%> (+0.32%)	89.00 <7.00> (+7.00)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.07% <0.00%> (-0.20%)	72.00% <0.00%> (-1.00%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)

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