Support text interchange with minsev severity

[MrAlias]

Fix #7576

Changes

Severity

• Add the String method to implement fmt.Stringer
• Add the MarshalText method to implement encoding.TextMarshaler
• Add the UnmarshalText method to implement encoding.TextUnmarshaler
• Add the AppendText method to implement encoding.TextAppender
• Add the MarshalJSON method to implement json.Marshaler
• Add the UnmarshalJSON method to implement json.Unmarshaler

SeverityVar

• Add the String method to implement fmt.Stringer
• Add the MarshalText method to implement encoding.TextMarshaler
• Add the UnmarshalText method to implement encoding.TextUnmarshaler
• Add the AppendText method to implement encoding.TextAppender

Tests

• Add tests for all added methods
• Add example test to exemplify JSON parsing
• Update example test for environment variable parsing to use the new unmarshal functionality

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.9%. Comparing base (49bf21e) to head (2711649).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7652     +/-   ##
=======================================
+ Coverage   78.7%   78.9%   +0.1%     
=======================================
  Files        186     186             
  Lines      14702   14819    +117     
=======================================
+ Hits       11582   11699    +117     
  Misses      2780    2780             
  Partials     340     340             

Files with missing lines	Coverage Δ
processors/minsev/severity.go	100.0% <100.0%> (ø)

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[sahidvelji]

Solid PR. Thank you! Left a few minor comments.