Use auto-instrumentation SDK in global tracing

[MrAlias]

When the auto-instrumentation attaches to a process using the global TracerProvider, and there has not been a delegate set, create a span from the go.opentelemetry.io/auto/sdk package so the OTel Go auto-instrumentation can instrument the application by default.

Resolve #5702

Benchmarks

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/internal/global
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                    │   old.txt    │            new.txt             │
                    │    sec/op    │    sec/op     vs base          │
StartEndSpanNoSDK-8   244.2n ± 21%   250.2n ± 23%  ~ (p=0.739 n=10)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.1%. Comparing base (b99d2b8) to head (ffbc715).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5920     +/-   ##
=======================================
- Coverage   84.6%   82.1%   -2.6%     
=======================================
  Files        272     273      +1     
  Lines      22883   23615    +732     
=======================================
+ Hits       19381   19397     +16     
- Misses      3157    3873    +716     
  Partials     345     345             

see 5 files with indirect coverage changes

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[pellared]

Discussed offline with @MrAlias.

We think that before merging this it would be good to:

1. rename sdk.GetTracerProvider to sdk.TracerProvider; reason: https://go.dev/doc/effective_go#Getters
2. make a v1 release of go.opentelemetry.io/auto/sdk