Drop the Extensions from Polly.Extensions namespace

[martintmk]

Details on the issue fix or feature implementation

Having Extensions in the namespace feels unnecessary. New namespaces:

• Polly.Extensions.Telemetry -> Polly.Telemetry
• Polly.Extensions.DependencyInjection -> Polly.DependencyInjection

Additionally, I have also moved Polly.Utils.LegacySupport to Polly.LegacySupport so the Utils namespace is not exposed.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

Merging #1469 (9db53ef) into main (3d8d6f4) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1469      +/-   ##
==========================================
- Coverage   83.89%   83.88%   -0.01%     
==========================================
  Files         275      275              
  Lines        6495     6492       -3     
  Branches     1015     1015              
==========================================
- Hits         5449     5446       -3     
  Misses        837      837              
  Partials      209      209              

Flag	Coverage Δ
linux	?
macos	?
windows	83.88% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...Core/CircuitBreaker/CircuitBreakerManualControl.cs	100.00% <ø> (ø)
...CircuitBreaker/CircuitBreakerResilienceStrategy.cs	100.00% <ø> (ø)
src/Polly.Core/LegacySupport.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategyT.Async.cs	100.00% <ø> (ø)
src/Polly.Core/Retry/RetryResilienceStrategy.cs	100.00% <ø> (ø)
...rc/Polly.Core/Utils/CompositeResilienceStrategy.cs	100.00% <ø> (ø)
src/Polly.Core/Utils/TaskHelper.cs	100.00% <ø> (ø)
...ependencyInjection/AddResilienceStrategyContext.cs	100.00% <ø> (ø)
...tion/ConfigureResilienceStrategyRegistryOptions.cs	100.00% <ø> (ø)
...ependencyInjection/PollyDependencyInjectionKeys.cs	100.00% <ø> (ø)
... and 14 more

... and 1 file with indirect coverage changes