feat: support accesslog substitution formatter in Literal type tag

[zirain]

make access log formatter (e.g. %UPSTREAM_HOST%) supported in Literal type tag

[arkodg]

just realized that the field has a type:Literal union discriminator associated with it, so reusing it for Formatter doesnt make much sense

should we deprecate the customTags field and use something like looks more like map[string]string ?

[zirain]

just realized that the field has a type:Literal union discriminator associated with it, so reusing it for Formatter doesnt make much sense

should we deprecate the customTags field and use something like looks more like map[string]string ?

CustomTags map[string]CustomTag `json:"customTags,omitempty"`

CustomTags is a map.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.33%. Comparing base (7864465) to head (d2bed3d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7640      +/-   ##
==========================================
- Coverage   72.33%   72.33%   -0.01%     
==========================================
  Files         232      232              
  Lines       34143    34141       -2     
==========================================
- Hits        24699    24696       -3     
  Misses       7669     7669              
- Partials     1775     1776       +1     

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