[WIP] Logs Bridge API

[pellared]

Fixes #3827

Based on #4725

Related to #4696

Benchmark results (all allocations are caused by usage of strconv functions):

$ go test -run=^$ -bench=.
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/log/internal
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
BenchmarkEmit/noop/no_attrs-16          450815065                2.617 ns/op           0 B/op          0 allocs/op
BenchmarkEmit/noop/3_attrs-16           75480518                15.22 ns/op            0 B/op          0 allocs/op
BenchmarkEmit/noop/5_attrs-16           58495515                20.10 ns/op            0 B/op          0 allocs/op
BenchmarkEmit/noop/10_attrs-16          39001635                26.68 ns/op            0 B/op          0 allocs/op
BenchmarkEmit/noop/40_attrs-16          12702404                81.41 ns/op            0 B/op          0 allocs/op
BenchmarkEmit/writer/no_attrs-16        15203751                75.06 ns/op           16 B/op          1 allocs/op
BenchmarkEmit/writer/3_attrs-16          5545542               209.2 ns/op            48 B/op          4 allocs/op
BenchmarkEmit/writer/5_attrs-16          4459609               244.9 ns/op            48 B/op          4 allocs/op
BenchmarkEmit/writer/10_attrs-16         2722802               410.5 ns/op            88 B/op          7 allocs/op
BenchmarkEmit/writer/40_attrs-16          779419              1371 ns/op             328 B/op         25 allocs/op
PASS
ok      go.opentelemetry.io/otel/log/internal   12.688s

This PR is to be split into multiple PRs to make reviewing more effective:

1. design doc: log: Add design doc #4809
2. source code and unit tests (update DESIGN.md to reference the source files)
3. package comments
4. benchmarks (update DESIGN.md to reference the benchmark test file)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (259143a) 82.3% compared to head (639b395) 82.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4798   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        226     228    +2     
  Lines      18401   18466   +65     
=====================================
+ Hits       15148   15211   +63     
- Misses      2969    2971    +2     
  Partials     284     284           

Files	Coverage Δ
log/provider.go	100.0% <100.0%> (ø)
log/internal/writer_logger.go	94.4% <94.4%> (ø)