add batchingOnly convenience method

[AlecZorab]

• similarly to batchingNotSupported, but for sources that only do
  batching (or for which batching is just as natural)

[codecov-io]

Codecov Report

Merging #110 into master will decrease coverage by -0.33%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   76.79%   76.47%   -0.33%     
==========================================
  Files          10       10              
  Lines         237      238       +1     
  Branches        2        2              
==========================================
  Hits          182      182              
- Misses         55       56       +1

Impacted Files	Coverage Δ
shared/src/main/scala/datasource.scala	66.66% <0%> (-13.34%)	❌

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[ghost]

Nice, thanks for taking the time to contribute!