Logs SDK prototype

[pellared]

Towards #4896

The design doc PR: #4954

Current benchmark results:

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/log
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
Benchmark/no_attrs/Simple-16     2481050               518.1 ns/op             0 B/op          0 allocs/op
Benchmark/no_attrs/Batch-16      2501084               516.1 ns/op             0 B/op          0 allocs/op
Benchmark/3_attrs/Simple-16      1379791               744.9 ns/op             0 B/op          0 allocs/op
Benchmark/3_attrs/Batch-16       1477051               865.1 ns/op             1 B/op          0 allocs/op
Benchmark/5_attrs/Simple-16      1322288               917.1 ns/op             0 B/op          0 allocs/op
Benchmark/5_attrs/Batch-16       1308534               865.2 ns/op             1 B/op          0 allocs/op
Benchmark/10_attrs/Simple-16      578826              2350 ns/op            1425 B/op          9 allocs/op
Benchmark/10_attrs/Batch-16       399573              2904 ns/op            1428 B/op          9 allocs/op
Benchmark/40_attrs/Simple-16      155461              7008 ns/op            6853 B/op         13 allocs/op
Benchmark/40_attrs/Batch-16       121132              8288 ns/op            6863 B/op         13 allocs/op

Side note:
There was one allocation if the context has no span. 29c90a0 I will submit a separate PR to fix it.

[codecov]

Codecov Report

Attention: Patch coverage is 43.54354% with 188 lines in your changes are missing coverage. Please review.

Project coverage is 82.6%. Comparing base (76921e9) to head (79ffb2d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4955     +/-   ##
=======================================
- Coverage   83.4%   82.6%   -0.8%     
=======================================
  Files        238     246      +8     
  Lines      15745   16108    +363     
=======================================
+ Hits       13143   13318    +175     
- Misses      2314    2482    +168     
- Partials     288     308     +20     

Files	Coverage Δ
exporters/otlp/otlplog/otlploggrpc/version.go	100.0% <100.0%> (ø)
exporters/otlp/otlplog/otlploghttp/version.go	100.0% <100.0%> (ø)
exporters/stdout/stdoutlog/exporter.go	0.0% <0.0%> (ø)
sdk/log/logger.go	91.6% <91.6%> (ø)
sdk/log/simple.go	89.4% <89.4%> (ø)
sdk/log/provider.go	52.0% <52.0%> (ø)
sdk/log/record.go	20.2% <20.2%> (ø)
sdk/log/batch.go	35.0% <35.0%> (ø)

... and 8 files with indirect coverage changes