[configtls] Add NewDefault* funcs

[TylerHelmuth]

Description:
Adds new NewDefault* funcs for all 3 config structs.

In anticipation of the name changes from #9495 I've named the functions using the new, preferred name.

Link to tracking Issue:
Closes #9657

Testing:
Added tests

Documentation:
godoc

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.95%. Comparing base (ae67b7c) to head (d3523ac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9658   +/-   ##
=======================================
  Coverage   91.95%   91.95%           
=======================================
  Files         357      357           
  Lines       16501    16509    +8     
=======================================
+ Hits        15173    15181    +8     
  Misses       1000     1000           
  Partials      328      328           

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[bogdandrutu]

Is there a need for this from users perspective? Or we think it is enough only the Client/Server? Happy to keep it if we have few needs.

[TylerHelmuth]

I dont have a specific user cases, only following the pattern defined in https://github.com/open-telemetry/opentelemetry-collector/blob/main/CONTRIBUTING.md#default-configuration. The pattern implies to me that we should have a NewDefault for all public config APIs. It is probably overkill, but it does allow us to add new default values to Config in the future if we want.

[andrzej-stencel]

I'd rather have the NewDefaultConfig function.

One thing is that we can use it in the NewDefaultClientConfig and NewDefaultServerConfig, although admittedly it doesn't need to be exported for this.

The other thing is that users can create their own configs that embed the configtls.Config struct and then they can use the configtls.NewDefaultConfig to create their instances of their configs.

I hope you don't ask me for potential examples of other configs that embed configtls.Config, because I don't have any such examples. 😄