[pkg/ottl] add string_converter

[led0nk]

Description:

• implemented func_string (string_converter)
• added new unit tests in pkg/ottl/ottlfuncs/func_string_test.go
• added function to pkg/ottl/ottlfuncs/functions.go
• updatet README.md at pkg/ottl/ottlfuncs/README.md
• added changelog

Link to tracking Issue:

• [pkg/ottl] Add String converter #27867

Testing:

• added similar tests as in pkg/ottl/ottlfuncs/func_double_test.go

Documentation:

[TylerHelmuth]

Please add a test case to pkg/ottl/e2e

[led0nk]

updated README - added e2e test cases for string-conversion and updated the function test :)

[led0nk]

accidentally deleted the branch, sorry for that 😅

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 82.11%. Comparing base (80ebb76) to head (008a0cb).
Report is 69 commits behind head on main.

Files	Patch %	Lines
pkg/ottl/ottlfuncs/func_string.go	47.36%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #31790      +/-   ##
==========================================
+ Coverage   81.94%   82.11%   +0.17%     
==========================================
  Files        1858     1873      +15     
  Lines      172674   174602    +1928     
==========================================
+ Hits       141490   143371    +1881     
- Misses      26892    26929      +37     
- Partials     4292     4302      +10     

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