feat: serialize AST to json

[sunng87]

Fixes #93

This patch attempts to add AST serialization to match output of prometheus 3.0 parse_query.

[codecov]

Codecov Report

Attention: Patch coverage is 98.63946% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.97%. Comparing base (4ea4cbf) to head (f7aed55).
Report is 1 commits behind head on main.

Changes have been made to critical files, which contain lines commonly executed in production. Learn more

Files with missing lines	Patch %	Lines
src/parser/function.rs	88.88%	1 Missing ⚠️
src/util/duration.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   98.98%   98.97%   -0.01%     
==========================================
  Files          14       14              
  Lines        6320     6467     +147     
==========================================
+ Hits         6256     6401     +145     
- Misses         64       66       +2     

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[sunng87]

Please let me know if there are promql statements have:

• subquery that has step is null
• function that has variadic true

[evenyag]

Others LGTM. I wonder if we should make some functions pub(crate) only.

[sunng87]

Sounds good. We should keep minimal access.

[waynexia]

Please let me know if there are promql statements have:

• subquery that has step is null
• function that has variadic true

AFAIK, subquery is a range query that doesn't accept a null step. And existing functions don't have variadic param list

[sunng87]

@waynexia @evenyag could you please cut a release for this?